Two day AF maybe causing me grief?

I have short cycles, 23-24 days like clockwork, even when I was on Mirena. When AF comes, it's very very heavy the first day and medium the second, and that's it. I don't even spot the next day, and this month was the first month I had any spotting before (it looked like implantation bleeding so getting AF was especially hard). Are my cycles and period too short?? I was using opk but never got a no brainier positive. The lines got darker but never as dark as a positive should be.