I get pain in penis during masturbation, after about 2-3 masturbation-sessions in a day. Never had this problem in the past. So, why this pain now?

Pain. Need more information but if no chance of infection or injury take a break to allow healing .
Overuse. If you do this 2-3 times regularly, the organ is telling you that it needs a rest. Try rubbing your elbow (which is a much less sensitive area) repetitively several times a day for a month and you will find that even the elbow will start to complain.