If you search online "pregnancy due date calculator" it will ask you when the first day of your last period was, You said it was March 11-15th. If you type in March 11th your estimated date of conception was 3/26, March 12= 3/27, March 13= 3/28, March 14= 3/29, March 15= 3/30 and so on. Could you have the date of your last period wrong or did you have sex with your boyfriend before before April 5th? If you got your period around March 21 then the estimated date of conception would be around April 5th, the day you said you had sex with your boyfriend. These estimates also depend on how long your cycles are, I entered the average of 28 days. The dates you provided aren't matching up exactly but either way i don't think it could be your ex's.
id have to say that its your boyfriends too. u had sex with ur ex before your period so i deffinitaly dont think its his. but if your worried you could always get a blood test or an ultrasound but thay just give an average of like a weeks time frame, i dont think they can pinpoint it to the exact day.
If you had a period after you had sex with your ex then it is not his. Since bleeding can occur during early pregnancy you'll need an ultrasound to date the pregnancy just to make sure. But it's highly unlikely that the ex is the father in this case. Did you take a pregnancy test any other time before getting the positive?