Very short period after years of long, heavy periods. Negati

Hello, I'm 21 and I've never been on any type of birth control. My cycle has been like clockwork for the last few years, every 30 days. My periods have always been on the heavy side. On day 30 this month my cycle started, or so I thought. The first day was lighter than normal and I didn't think much of it, but by the beginning of the second day it was really lite. By the second half of day 2 it had stopped and hasn't started back up since and it's been 3 days. Each time my husband and I had sex this month we used a condom and none of them broke. I took a pregnancy test 2 days after my period mysteriously ended and it read negative.

My question is could it be a combination of a few things happening in my life that's affecting my cycle? I've started to exercise 6 days a week for at least 40 minutes, I started taking a fat burning diet pill and hoodia. During this last week my sleep schedule went from me waking at 8 am to me not being able to fall asleep until 6 am. Could it be stress? Could I be pregnant? I honestly don't see how I could be, so what's GOING ON?!!! Help me, please!