thanks ladies you both have been really helpful.
Most likely you weren't entirely done with your period and having sex has made what was left from your period to be expelled from your body.
Can I just correct what the above poster has said however, sperm can live (in healthy conditions) 5-7 days at most inside you, and so if you ovulate early then you could well end up pregnant. Please use condoms any time you have genital to genital contact to prevent pregnancy and STD's.
You could just be bleeding out left over blood thats been in there, it happens fairly often. Try a back up method of birth control as sperm are pretty hardy and can live a good 72 hours in there.
Try the morning after pill.
I wouldn't really worry about getting pregnant this early on after your period, it can happen but it is highly unlikely.