What Causes Spotting And Stomach Bloating With Back Pain?

I'm a 18 year old female and I haven't had my period in 5 months and they use to be irregular and then I started loosing weight and it got normal and now I haven't got one in 5 months. I spotted lightly a few weeks ago. My stomach keeps getting harder and bigger, but the rest of my body is getting smaller. I've had back problems before and now they are worse. I keep getting this pain in my stomach like someone is kicking. I felt it and something was there it freaked me out. I had my hand on my lower belly and I felt heart beats and I keep feeling them. Please help me. I've been having dreams lately about being pregnant.