Any ideas?

My period was supposed to start (9/15), but it came on 4 days early. It came on(9/11-12) and it was somewhat heavy. Then it was light... The night of (9/13), I didn't bleed at all. I had some spotting on (9/14). None on (9/15). I took a pregnancy test Monday or Tuesday.. and it was "Negative". I've been having some sharp abdominal pain, constipation, gas, lower back pain, and etc..in the mornings if I eat anything, I get really nauseous. I haven't been wanting to eat much as well. I don't know what's going on.