After sex with my bf my vagina became very dry and a little itchy and sore we have no other sex partners.this happens occasionly after sex sumtimes?

Several possibilitie: In today's world, so many women are hypothalamic. Typically thin, triplea personality, with history of excessive aerobic exercise or very low weight etc. These women have much lower estrogen and end up with vaginal dryness much like menopause (atrophy). We see many such pts and ocp's makes it worse. Need vaginal estrogen high dose. Otherwise need rule out infections like bv, trichomonas etc.