So upset, dont understand why this has to happen?

I ve took two preg tests an they gave me the impression that i was expecting. Now im bleedin heavily with few clots and with pains which sorta feel like a period, but how would i get a positive result on a preg test. this must be a miscarriage or is it?? And if it is how many other times have i thought ive had a period and its been a miscarriage? Could this be linked to the fact that i need to have another mmr jab becos my body isn't producing the immunity to rubella.. i dont understand why i cant fall pregnant. Please only reply with useful information from knowledge and own experience.