Hi, Nervous, don't worry, we don't judge here in the DNA/Paternity forum.
Apps are based on averages and assume consistency -- that the woman ovulates and has her periods exactly so many days apart, month after month. You can't count on your app to tell you when you ovulated if your cycles are not like clockwork.
This means the first question to focus on is whether you in fact got pregnant from the sex on March 7. If it was the only sex you had in February and March, well, you did. But if not, you might be focusing unnecessarily on the lively and interesting events of March 7 when you actually got pregnant before March 7. In fact, March 7 sounds too late (given what you know at this point). Did you have sex with anyone in the last week of February or the first week of March (not including the 7th?)
If you did have sex before March 7 (but not too long before), one way to check things is to see an ob-gyn right away, before any more time goes by, and get an ultrasound. This would be your best play at the moment but you have to move fast. Using an ultrasound to try to determine the possible conception date is time sensitive. Embryos begin as one cell and then divide at a known rate for a while -- the embryo's measurements can be spot-on for its age in your sixth or seventh week (counted from the first day of your last period) -- but after a while they can grow faster or slower, and ultrasounds measuring the baby get less and less accurate for determining a conception date as the pregnancy progresses. (By your 12th week, the doctor will say "give or take 7 days," which is unhelpful when you want to split the difference between, say, March 3 and March 7.) Anyway, if you had sex with anyone in late February or early March (other than the 7th), call your doc and see if he or she will order an ultrasound, and get in for one right away. Pay cash if you have to, but get it.
Otherwise, (and this also would be true if you didn't have sex at all in February or March except for March 7), there will be no way to tell who the dad is without a DNA test. If you are in that spot, you could pay the big bucks to Ravgen or the DDC for a non-invasive prenatal DNA test, or wait and pay about a tenth as much for a test after the baby comes.
I hear you saying that one guy used protection and the other didn't, but unfortunately condoms are only 80%-90% effective depending on how they are used and how early they are put on. So it is not possible to rule him totally out. For that reason, if you get down to a DNA test, you will need to test with both guys.
If you get in for an ultrasound, either tell the doctor that your cycles are too irregular to use as a start point for counting the time period of the pregnancy, or just say you don't know when your last period was. That way, any numbers you get from the ultrasound will be certain to be from the embryo's measurements and developmental markers only. Be SURE to ask the tech or the doctor for a due date (estimated due date, or EDD) from the ultrasound. When someone's cycles are irregular, it gets VERY confusing to try to compute things from an ultrasound tech just chirpily saying you are a certain number of weeks and days, because you don't know when they began the count of weeks. (*Hint, they don't begin it at conception.) So ask for your due date. Then bring it back home and plug it into a conception calculator online, or just count back 266 days on a calendar. That would be your estimated conception date. Again, it's very important that you do this soon, or it won't help you.
Write back if you get an early ultrasound. We can try to puzzle out the results.