It's hard not knowing, but I think the ultra sound is accurate, especially if you are a regular person with a 28 day cycle, which is ideal for the standard due date calculations. It looks like you ovulated one week after your period started, which really makes sense: a lot of women are that way.
If you were sleeping with 2 guys in the same month, it's going to be almost impossible to tell. If you were only sleeping with one guy since you think you got pregnant (the 14th of March by the calculations) then it's whoever that guy is, assuming that was when you got pregnant.
But really only a DNA test is going to tell who whose it is if you can't figure it out. Some guys don't have a high sperm count or any viable sperm and it could be the case with one of them. Or something else could have happened. That's why, I guess this is obvious for you now, that you stick to only one partner and if you "break up" with someone, you better make sure it's permanent before you start something with someone else. So many people on this board have this problem of not knowing who the father of their baby is because they quarreled one day, got with someone else the next day, and then got back with the first guy the next. Too late!
DNA tests are simple and easy to get, but only after the baby is born. You are going to have to tell your boyfriend the truth about what happened, and if he loves you, he will stick it through with you. If he doesn't, he's not man enough to be with you or be a good father.