How long are you periods normally? Your period gets lighter after you've been on birth control for awhile. I've been on birth control for six months now and my period is only three days long so what you had sounds like a normal period to me.
Yes, it's possible to get pregnant, but given your circumstances it's highly improbable. Just birth control alone is 99% effective and you're not ovulating during your period.
If you're still nervous even when on birth control, use condoms and birth control, that way you'll be
really safe