After masturbation, my entire pelvic area hurts, as well as inside my vagina (only sometimes). what could be causing the pain?

Endometriosis: It is not uncommon for the uterus and related genito-urinary structures to undergo "contractions"/spasms in the resolution (post-orgasm) phase. This is assuming that orgasm was achieved. Typically, such post-orgasm changes can be sensed as cramping; however, in some situations where endometriosis is present, the uterus and related structures may be scarred down and mobility limited with pain