If your period had started 11th April, then it would have probably suggested more certainly that your bf would be the dad. But that would probably change all the other dates. However, it didn't start 11th, but 18th.
Why do you ask???
Yes it's still most likely your boyfriend. You had a normal period AFTER you had sex with "some guy."
You will NOT have a normal period if you are pregnant. There can be some bleeding however.