What should i do when there is pain between biceps and forearm curling?

REST.: You could be developing tendinitis, strain or partial tearing of the biceps insertion in the upper part of your forearm bone (where the bicpes inserts onto the radial tuberosity). Avoiding bicpes exercises or reducing the intensity would be a good start. Antiinflammatories, ice to the area may be helpful adjuncts. If pain persisits see an orthopedic surgeon.