What Causes Spotting,nausea,fatigue And Back Pain ?

I am an 18 year old female. I just recently discovered that I am no more than six weeks pregnant. Everything is normal (that I can tell) except one thing. Everytime that I have sex I bleed from my vagina like I'm starting my period, but it either slows down to spotting or stops completely. I have stopped having sex and I'm still spotting right now. I have symptoms like nausea and fatigue, and sometimes I have back pain, but I also have minor scoliosis, so that should be normal.