Did you have a scan at week 12?
Have you had a 20 week scan?
Having a normal period after intercourse is very suggestive that the pregnancy did not occur due to the time that you cheated on your boyfriend. If your due date is 8th June and was confirmed by scan then this means that you conceived in the middle of September therefore that would be your boyfriends baby.
However to be absolutely sure then you would have to do a DNA test.