You say your periods come every 31 days and put a lot of faith in your computations based on this, but then say you always have a period on the 29th of every month. Both things can't be true -- April, June and November have only 30 days in them and February has 28 or 29; if your periods always come on the 29th that would mean you are irregular, not regular. And remarkable, too, since it would mean your periods can read a calendar and tell if it is April and not May, and vary the cycle length to always hit the 29th of the month. So, the first piece of feedback is don't be so sure your period arrived on the 29th, especially if you haven't been keeping track. You are using this to think what must be wrong is a medical test (ultrasound) instead of your assessment of when you must have ovulated based on a cycle month that merely assumes your period began on the 29th. It is possible that your periods have shifted slightly in relation to the calendar, if you haven't been charting them.
Can you give dates, not just analysis? When did you have the sex with each guy, when did you have the strong pain, when did you have the ultrasound? The only date you mentioned is your estimated due date, all the rest is interpretation or only relative to the other incidents in the story. Your interpretation might be perfect, but there is no way to answer your question without the data. .
One other way to interpret the data is that on the 14th you ovulated and on the 19th the pain you felt was implantation. Not that this ties in with the medical evidence from the ultrasound, however. I'm just mentioning it to back up the fact that you do need to do the DNA test with both guys.