"Another guy" is the father from what you posted.
1. You won't have a normal period if you were pregnant.
2. Doctors base dates on your LMP. It works for March 28.
You had sex on day 10 of your cycle. You'd normally ovulate between days 11 and 16 so you had sex almost at peak fertility. It can take several days before the egg is fertilized and implanted so you conceived around April 11. No one can tell you the exact moment.
Hope it helps.