Recently during sex it feels like i've been kicked in the testicles. what could be causing this?

Could be: An infection of the prostate gland or spasm of the cremaster muscle which control how high or low the testicle hangs in the scrotum.

Inflammation: It seems that you have tender testicles that hurt during intercourse. Infection (like chlamydia), mumps, trauma to the testicles and prostatitis can cause tender testicles. Referred pain from the ureter and the pelvic organs, inguinal hernia can also cause referred pain to the testicle. A careful history and physical exam can usually identify the cause that can then be treated appropriately.