"I had unprotected sex with my boyfriend a week after my period finished. But there may have been semen left on his penis. Could I be pregnant?

Yes, of course.
Unprotected sex is unprotected sex, and I'm not sure you wrote what you meant, because it doesn't quite make sense. But regardless, you can ALWAYS get pregnant.

Yes of course, your post made no sense, since he would have sperm when he had unprotected sex with you

If you had unprotected sex then there wasn't only sperm left on his penis, there was sperm put directly inside of you.

Any time you have sex, even if you use protection, pregnancy is possible. So yes, it's entirely possible that you're pregnant.