Suggest Treatment For Painful Retraction Of Foreskin On Penis

i am 24 yrs old. when i hav sex with my girl friend my prepuce retracts back and causes pain . i dont have any pain with my penis in normal non erected state and when i masturbate without prepuce sliding back. i can clean normally retracting my prepuce back. but i have pain when penis is in erected state .i think this is not phimosis as i can retract fully the prepuce. suggest me solution