Brown gooey discharge instead of normal period, two negative

I had sex on feb 21st and havent since but the week following i vomitted once and had a slight thought that perhaps i was pregnant. Two weeks after sex I took a pregnancy test and it was negative. Then in the week following (third week) when my period was due, it arrived but it was very dark brown, almost black, and very thick. It lasted for around 2 maybe 3 days. I was quite scared but after the test I thought that I couldn't be pregnant so I waited for my next period. When it came it was the same but lasted longer for around 5 days. I agen thought that it was my body adjusting and thought to wait one more time, however I got quite scared and took another test. It was negative again. When I was scared and worried, very stressed, I began to get pains in my breasts and they were very sore but when I took the second test and became calm my breasts stopped hurting and my period came that day. I feel calmer that I couldn't be pregnant and I have spoken to a nurse and she feels it may have been a miscarriage causing my period to be awry, however is there a possibility that I am pregnant or do I have an infection.
I have not had any symptoms of being pregnant.