My question is, if i have a regular 28 days cycle and had unprotectedsex 15 days after my period am i at higher risk of becoming pregnant?

Fertility: That would probably be around the time of your expected ovulation, so yes. If you don't want to get pregnant, then please use reliable contraception.

Probably: When I think about fertile days, I count from the first day of your period. The last day isn't what we use. So, if by day 15 you mean from 1st day of that cycle then yes. If after it ended, a little less likely though still possible. Also, cycle length is from start of one period to start of next.