You had a heavy bleeding--a period--so the chances that you are pregnant are very, very small. Very few women have full-blown bleeding and go on to be pregnant. Second, you would not be showing so early.
I'm 22 weeks pregnant (they set the due date at 40 weeks, so I'm halfway there) and still just barely showing. You look at me, you still wouldn't know I'm pregnant. I didn't start to get anything like a tiny belly until I was 14-15 weeks along (that is 10-11 weeks after the first missed period).
But you should see your doctor to determine what is wrong.
Have you been having unprotected sex? If not, the likelihood of pregnancy is very, very small. If you're not ready for children, start using condoms or get on the pill. Getting on the pill should help regulate your periods as well so they come on time and often the pill makes your periods lighter too.