Positive HPT but bleeding?

Ok so I got my period on the 30th of March. Had during ovulation. 3 days ago I took the first response early detection HPT. In about 30 seconds it showed a faint positive. Today is the day that i should have gotten my period and guess what..its here! So why do people say that pretty much anytime you get even a faint positive you are pregnant. If that's the case then why am I bleeding? Its not heavy right now, just light spotting but im sure it is going to get heavier. I have the cramps and everything else accompanied with my period. So my question is...is first response a bad test to use? I know this is not a miscarriage because i got my period right on time.