Well, from the first due date they gave you, you didn't conceive until mid-April, unless your due date was wrong.
There is one rarity in the medical literature that might explain it, but another thing that is much simpler that might explain it is that ultrasounds done late in pregnancy can be as much as two weeks or three weeks off, due to differences in the growth rate of babies.
So if your due date was given to you later in the pregnancy, that could explain it. This wouldn't stretch things back to February, though, only to the end of March.
In case the rarity in medical literature (which was treated as being rather crackpot when it was published) applies to you, why not just get a DNA test with both guys? That should answer the question once and for all. (Unless they both come out "no." Then you have a different kettle of fish.)
No end of March but I was also on the contraception pill
Hi, did you sleep with anyone in April?