pregnancy bleeding.

Is it possible to have medium and a couple of heavy days bleeding for 5 days every month when due for my period and still be pregnant? 
I'm confused because some people say they have bleeding through their WHOLE pregnancy. so it's freaked me out a bit. 
Just wondering if this bleeding is heavy or light when pregnant and was it every month? 
I haven't had sex for about 4 months (start of February)  and I have had 6 periods since then. Have taken pregnancy tests and they have came back negative. I have been having symptoms but no positive tests and the doctors said I'm not because I'm getting periods. 
Help!