What Causes Vaginal Bleeding?

I had blood coming out of my vagina. I am pretty sure it wasnt my menstruation, because i just got it around a week ago. The blood is not like when u're having period. It's more like a pale red and brownish red. The blood coming out was not all the time. Sometimes, it doesnt come out at all. Sometimes, it just comes out. But when it does, its not that much of blood coming out. Even when I am using pantyliners, the blood didnt went out of the pantyliners. My stomach is also in pain. This has been going for about 2 days. I'v been having sex with my boyfriend. Is it related to this case? I'm really scared it's a sign of something bad. What should I do?