Random bleeding. HELP!! ?

Every time my boyfriend and I have sex or just muck around, I always seem to bleed down there, its bright red and some times bleeds for a little while, I had my last period 2nd June. I sometimes get really bad stomach aches during the week, but that has been happening long before this, we are sexually active but always use condoms. I'm 21, is this anything to be concerned about? Really worried.