Listen to your body when it is trying to tell you something. The sooner the symptoms are recognized and treated, the sooner and more completely recovery can occur. Overuse injuries can be categorized on a scale from one to five, five being the most severe.
1) Pain at one site and only while playing. The pain ceases after playing.
2) Pain at multiple sites when playing but there is no pain outside of music-making.
3) Pain persists when you are away from the instrument. Player underperforms.
4) All of the above where most aspects of hand function cause pain, even at rest and at night. Player is only able to continue with stoicism.
5) All of the above but all daily activities that engages the affected body part cause pain and there is very little capacity for playing.
But there may be more subtle indicators than pain! There may be burning or a "hot" sensation, fatigue or heaviness, weakness, impaired dexterity, numbness or tingling, clumsiness or stiffness, involuntary movement and loss of coordination without pain. Keep a daily practice diary and note if something was sluggish or more difficult than before - something you could "nail" no problem a few weeks ago and today your fingers just won't do it. Try to look back and to analyze what you may have been doing differently: more practicing? a new instrument? a new technique? a more demanding conductor or teacher? Perhaps you played in a cold environment with poor chairs and inadequate lighting? Did you help a friend move and/or paint their apartment? Did you spend hours on the computer typing a paper (or texting) and practice a lot?
These more subtle signs indicate that this is a time to back off and stop playing for a few days. Notice difficulty with normal daily activities and get help immediately if you feel numbness for more than one day. Playing (Less) Hurt has an extensive resource list of qualified health care professionals who understand musicians' issues. Don't wait until problems become severe. Complete recovery is possible and the lessons learned about your body's limitations are invaluable.