Period Stopping Suddenly

[caricaneba15.livejournal.com]

Yesterday I was completing the 4th mile of my run, when I felt a horrible side stitch. I had to stop running for about 10 minutes until I could recover.I usually never get these, which is what made me think it may have been period related. After that,my period stopped completely.It was pretty heavy flow before though. This is rather weird because usually I'll have some spotting, and it will last longer than 2 days.I'm still having cramps...and I took a blood pregnancy test a couple of days ago, and it was negative, so that is ruled out.any ideas on why this happened?