I have to admit that I am no health professional but to me it sounds like you are just trying to climb harder that your muscles allow after your climbing break.
The problem with restarting any physical activity after some time of being away from it is, that your brain and the memory of your motor system still know how to do things but your muscles, tendons, ligaments, and joints have lost strength. Therefore you are very likely to just overdo when starting an activity again. Especially with climbing there are some things where you are able to compensate a lack of strength with some bad habit techniques that are prone to give you some bad injuries.
So first of all I would think that you are maybe just climbing harder than your muscles allow at the moment. Also not warming up your fingers enough could influence your muscle performance. Maybe you have also lost some confidence in the belay gear and therefore crimp more than you would have to.
In any case I would recommend to just take it easy for the next times of climbing until you have sort of regained your strength. That means, better take some really easy routes for warming up and then increase the grade slowly. Especially if you feel that your grip strength is reduced don't try to compensate with weird hand positions or the like.
If your muscles start to cramp better stop climbing for that session or at least take some hours of break. As you have to cycle home after climbing, better stop climbing a bit earlier than at a point where you already cannot open your fist anymore.
