I briefly lifted a heavy bag by its drawstring, and ever since then one side of my middle fingertip is numb. is it a pinched nerve? how do i fix it?

May be: It may be parasthesias caused by a nerve injury you sustained when picking up the heavy bag and irritating the nerve that innervates the tip of your finger. Nerve injuries can heal over time, even more so if properly diagnosed and treated quickly. Best for you to see a dr, get eval and testing (possible emg) so the best treatment can start. Good luck.