I had my period on the first of Dec. & I had unprotected sex on the last day of my period which was the 6th . My period came again on the 28 of Dec. Could I be pregnant ?

I assume you were trying to get pregnant since you had unprotected sex? I don't think its likely because you had another period... but there are things called false periods.. so the next question is did you take a test?

Very unlikely, but slightly possible. You will have to wait and see if you miss next month or not