I had unprotected sex with my boyfriend around the end of the month last month, & I usually start my period around the 12th of every month. I went to the doctor about a week agao for a UTI & was told I wasn't pregnant. It's now the 21st & still no period, could the doctor be wrong?

Doctors and machines aren't perfect. Errors can occur, but that doesn't seem right for you to still not get your period. Get tested again and it will be more accurate.

The doctor did a pregnancy blood test ?

But they can be wrong, you can go and get re-checked