Ok there are a few things it could be:
1. maybe sex was a little more rough than usual yesterday?
2. did you just get done having sex or have you today (its not marked on FF...so just asking)? it could be from that.
3. Maybe AF spotting (I dont think so this early though!!)
3. OR the kicker, it could be implanation spotting
Dont panic, just keep an eye on it!