What Causes Spotting Followed By Heavy Bleeding After Periods?

My husband and I have been trying to get pregnant for a year now. I missed my period last month but the pregnancy test said neg. I got my period on the 3rd of this month and it was normal. It lasted 4 days. Yesterday when I used the bathroom there was some blood on the toilet paper. I didn't have any cramping or pain. It continued each time I went to the bathroom. I called my doctor and he told me to come in for a blood test to see if I am pregnant. Last night I had to use a tampon because the bleeding is very heavy and dark. What is wrong and could I be pregnant?