Any ideas?

I have very consitent periods...always have. AF was due yesterday and on Monday and Tuesday I had very light spotting...from everything people have said on here I was almost certain it was implantation blood. Wednesday came and no sign of AF. I was really getting my hopes up even though I was telling myself not to! Wednesday night about 7:00 I started getting pretty bad cramps (worse than I've ever had...I usually have really light cramps) and then shortly after AF arrived :( any ideas what the spotting could have been? I've never had it before.