Why Am Getting Pregnancy Symptoms But With Negative Results?

The thing I noticed that was strange, was the fact I had my period for only a day and a half. I woke up the first day, in the middle of the night with horrible cramping pain. And was prescribed Loratab, and Ibuprofen 800mg. After it ended, I've had light cramping where I would if it were period cramps. I've had lower back pain, and a loss in appetite. I just want to know what might be wrong. I also took a $1 pregnancy test, and it was neg.