Tightening in stomach

My periods are finally regular and yesterday i had nothing but clear watery discharge like alot and now today im having these cramps like tightening in my stomach its almost a little different from period cramps its in my lower stomach and around my belly button. Im not due for my period til 7 days and i think i remember having this pain during my first pregnancy but i miscarried at 6 weeks last year. Im just a little worried if it is pregnancy im gonna have to get the rhogham shot for being o negative. Please help !!!