Injuries in tennis most often occur from 1.) overuse and 2.) technique. Playing too much, especially combined with technical issues, can lead to pain and eventually an injury. String and racquet choices can lead to injuries sometimes but usually poor string and racquet selections lead to bad technique which is the real reason for the injury. For example, if you use a racquet or string that is not powerful enough or too stiff, you will try to over hit and therefore your technique will break down which can lead to an injury. Therefore, switching to a more powerful racquet or string can help sometimes with pain — but usually the help comes from improved technique and not needing to generate as much power on your own.
The first question with any injury is — will continuing to play tennis going to make my injury worse? If that is the case, then rest or some form of medical treatment is necessary. If the answer is no and continuing to play will not make the injury last longer, then try to find ways to endure the pain and continuing playing or training. More often than not, a few days of rest will do the trick and is far better than pushing a potential injury and making it worse which might require an extended absence. If you do rest and find little or no relief then you will need to explore some form of medical treatment.
It is super important to remember to do all that you can to help prevent and endure through injuries. For example, you should start any form of a practice session with a proper dynamic warm up. You should finish any session or day with a proper stretch. Usually at the first sign of pain, you should make sure to spend extra time stretching, icing, and strengthening the area in pain. Also, proper hydration, nutrition, and sleep will help with preventing and enduring injuries.
Speaking of REST…how much should you rest in general? 1-2 days/week depending on your goals. After a tournament you should take a day off IF you have a tournament the very next weekend. If you do not have a tournament the very next weekend, you should come to practice as soon as possible and work on playing fatigued/multiple weeks in a row. Then you can rest Saturday and Sunday the following weekend. If you are not in a good place with school or sleep, you need to take sometime to get caught up in those aspects. You cannot practice properly overly stressed about school or sleep deprived. Lastly, do NOT rest if you are sore! Sometimes soreness is confused with injury. When you are sore the best thing you can do is work out the soreness through exercise.