Missing period. Could it be something other than pregnancy? Exercise?

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 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 4 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 3 days after my period mysteriously ended and it read negative and I'm going to take another one tomorrow.

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? What's going on? Thanks for any help.