What Causes Intermenstrual Bleeding And Abdominal Distension?

I have had unprotected sex several times with my boyfriend. The missed opportunities, he has also come to me but I have eaten since the morning-after pill. I have always done pregnancy test which proved negative. This month my period was delayed for eight days. But before that I had already had a bleed a week after I already had my period. I have never had any bleeding between periods. However, I have been very stressed out at the last evil since I graduated from high school last week. but I should check so all is well? My stomach feels sore and weird and I imagine all the time that the stomach has become larger. I have never tested myself against sexually transmitted diseases, should I?

Excuse my bad English, I am using google translate