Here are more symptoms typically associated with overtraining syndrome (OTS):
1) A highly noticeable plateau or decline in workout performance or progress.
2) A perception of increased exertion during workouts, even early on when they aren't particularly tough.
3) Excessive and abnormal sweating or overheating.
4) Unusual feelings of heaviness, stiffness, or soreness in muscles.
5) A lack of feeling “refreshed” after regular rest and recovery.
6) Recurrent injuries, such as muscle sprains, tendonitis, stress fractures, and chronic joint pain.
7) A decline in enthusiasm for exercise.
8) Persistent feelings of fatigue, exhaustion, or low energy throughout the day.
9) A decline in motivation and/or self-confidence.
10) A lack of enjoyment in favorite hobbies and interests or other signs of depression.
11) Unusual mood or emotions, such as agitation, anger, confusion, irritability, and restlessness.
12) New problems with sleeping, including insomnia and poor sleep quality.
13) Problems with concentration and performance at work, school, hobbies, etc.
14) A sickly appearance, including changes to skin, hair, and nails (such as acne or hair loss).
15) An increase in resting heart rate and/or resting blood pressure.
16) Unplanned/undesired weight loss or weight gain or disordered eating.
17) Newish digestive issues, such as constipation, diarrhea, loss of appetite, and increase in thirst.
18) Decrease in libido and a change in menstruation (including irregularity or cessation of periods).
19) Repeated bouts of illness, such as colds and upper respiratory tract infections.
It is very possible that you could have many of these symptoms and not suffer from OTS. There is currently no test that can definitively diagnose it either. Sometimes these issues can come up for reasons other than overtraining too, like other illnesses, injuries, or even just significant life changes so make sure to consider all factors if any of these things are happening to you.
But if it is related to overtraining, the good news is that it is pretty easy to fix with pretty obvious fixes - rest, recovery and nutrition. These things are important regardless but they may require more time when dealing with OTS. Getting 7-9 hours sleep every night consistently, not just once in a while, and making your sleep as comfortable as possible. Rehydrating and refueling properly after strenuous exercise. Stretching, foam-rolling and massage help too. Cut down on the intensity and frequency of your physical activity. Making sure you are following healthy eating habits. If you really do have OTS, you won't notice a change right away. I've heard it can sometimes take months in some cases.
As far as resting between exercise and training sessions, it depends a bit on who you are and what you do. When it comes to boxing, I have found that you can do it almost every day but you really need to listen to your body. In terms of whether you decide to train that day and how intensely you want your training session to be. Just about everyone should take at least one rest day. Professional boxers in camp right before a fight, usually never go for more than 6 days without a day off and again, the intensity level and even specific training can vary.
It is important that your rest day really is a rest day. This can be harder than it sounds if you are pretty active in that your mind can play mean tricks on you. It can make you feel useless and lazy and like you ate too much and if you don't do anything to "correct" this, either that day or the next training day, you are going to lose what you have worked so hard for. Be very careful of this voice, especially if you train a lot and eat generally well. It can lead you down the path to a distorted reality, which leads to other problems like eating disorders as well as overtraining syndrome. Enjoy your days off, eat properly and remember that sometimes doing nothing is exactly what you need.
Another thing that can be tough for some of us is drawing the line between working really, really hard and pushing yourself vs. potentially overtraining. If it doesn't challenge you, it will not change you so if you want to see results, you will have to work harder but not so hard that we start to move towards OTS or potentially cause some other injury. This line is different for all of us. You have to just be really mindful of both sides and be willing to experiment a little.
You do not have to do any of this alone either! Fortunately, significant OTS is pretty rare with the vast majority of people, particularly those of us who primarily just box, but if you are worried about yourself, seek out some professional help - a coach, a trainer, a nutritionist, your doctor, all of the above. Keep a journal of how you are feeling before and after workouts so that you can share more detailed information with your wellness team.