Caveat: I'm not a doctor, a nurse, or anyone else trained in anything remotely describable as "medical". This is what I've found that works for me.

Ongoing pain like that is not a good thing.  I'd suggest cutting your practice time down significantly (to 3/4s or even 2/3s of what you've been doing) for a few days, and see what happens. If the pain and trembling goes away, then I'd say yes, you've been overdoing it.  If it's better, but still there, you might need to cut back even more (for a while - you can build up strength and endurance, but it takes time.) If there's no change, then I'd say see a doctor.

I don't know of any specific or official exercises. What I always do for any muscle that is simply tired is stretch it.  Both hands are being pulled in, like making a fist. So pulling or pushing the fingers out should help. Try putting both hands against each other, fingertip to fingertip and (gently!) push the fingers back. A "cracking your knuckles" motion may help, too.  (They recently proved that that does NOT give you arthritis, in case you're worried about that.)

I occasionally get caught up in something and play too long, and when I finally untwist my left arm, it lets me know in no uncertain terms that it's unhappy about it. Both with the muscles of my left arm and with some assistance from my right hand, I stretch it by twisting it in the opposite direction.  Massage helps, too.

The main reason stretching the muscles in the opposite direction helps (to the best of my knowledge) is that the body mostly has muscles in pairs. When one muscle is being used, it bunches up, which restricts blood flow.  If you use only that muscle (and not its opposite) for a long time, it's undernourished and fatigue poisons build up.  When you use the opposite muscle, it is the one that has the restricted blood flow, so the blood has to go through the other one. That's why track coaches insist on runners kicking their feet up high. It doesn't improve their speed any, but it gives the muscles in the front of the thigh (that are doing the lion's share of the work) a blast of food, oxygen, and cleaning out. They get fatigued a lot more slowly that way.

It's not practical when playing a violin to throw your fingers way back in the opposite direction, but stretching them out (and maybe dropping your hand down and shaking it to help get blood back in it) during breaks will help some.  You can't take frequent breaks in a performance, but you CAN do so in practice. You might try that, too.

Glad it was an easy fix.

I don't know what your practice routine is, but if it's one long session now and your schedule allows it, break it up into several smaller sessions, and that will help a lot. Both your body and your mind will do much better with short, comparatively intense, focused periods of work, then a chunk of time (preferably at least a couple of hours) of relaxation from that type of work (which might not look like relaxation to an outsider), then another short, fairly intense session, and so on thru the day.

Mentally, it lets your subconscious sort through things and figure out how to integrate some of the stuff you just did. Physically, you work other muscles, and get some blood flowing through the ones that were being worked.  Long sessions of anything are bad for human beings.  (Says the man who works a 12 hour shift with no breaks or lunch...  This addiction to having an income is killing me...)