Is this normal?

It's been 8 weeks, and I've had my period for the third time. Now the first menstrual cycle lasted about 3 days. And in the past few months, my period has been lasting 3-5 days, so I thought it was just he same as any other cycle. The second cycle started about a week and a half after the first cycle, it was quite heavier and lasted about 7 days. So I thought maybe my body is finally going back to it's "normal" cycle (which was 5-7days since I began having menstrual cycles up until the recent months), or that my body may be reacting to stress from running 3miles/day. That was until I got my cycle for the 3rd within the last 8 weeks, which yet again, was about a week and half after the last cycle. The 3rd cycle, was just as heavy as the 2nd, and lasted about 8 days. It's been about a week since that period, and I'm beginning to have that "warning discharge" before my cycle starts, and I have a strong feeling that I would be getting it yet again.