Also, if your last period was May 7, get that ultrasound this week or next. I can't exaggerate how helpful it will be to have a sixth week or seventh week ultrasound and how unhelpful it will be after your 8th or 9th week. This is the time to get one, do everything you can to get one and to get an estimated due date from the measurements of the baby. (A calendar date. Don't just let them tell you a weeks-pregnant figure and pat you on the head and send you away.)
Hi, first thing is to work out if you really did get pregnant from that eventful day. You might be going through all these worries for nothing.
This doesn't really solve your problem, though. The fact that you had sex with two guys and are pregnant can be figured out. The problem that is putting the fuel to the fire on the question of whether the wrong guy is the dad is the fact that you are in a relationship with a guy who doesn't keep promises of fidelity in relationships and you find that unsatisfactory. I would find that a deal breaker myself, since that is who he is -- he is not going to change his opinion of such behavior being acceptable for him to do given the circumstances, no matter how long he is your boyfriend or whether you have a baby or not. Do you have a job and a stable life, and your own place? Or are you dependent on him? Because you really have some deciding to do, whether the guy is worth it or not, separately from the issue of any baby. And if the baby is from the wrong guy, you've got to get your ducks in a row fast to create a self-sustaining life.
And just so you know, if we can figure out if the sex with your ex was in the right time range, nothing you did in the bedroom with him makes it less likely that he is the dad. Sperm lives in a guy's urethra after an ejaculation for quite a while, and you guys 'putting it back in' after he ejaculated means his pre-ejaculatory fluid from the second erection carried sperm into your body pretty much at the first poke. If you were ovulating then, or if you ovulated in the next 5 days (sperm lives that long), you could certainly have gotten pregnant from your ex.
So, let's figure out whether there is a chance that you got pregnant from the interesting events of May 16. It all depends on when you were ovulating. Are your monthly cycles super regular? Had you just had a period, or were you expecting your period? What makes you think May 16 was the day?
If you think May 16 is likely, you need to make an appointment with your doctor. You need to get an ultrasound, make the appointment for about six weeks after the first day of your last period, if your cycles are regular.
Make sure you can get an ultrasound, when you call for an appointment. If your doctor doesn't do ultrasounds in the early weeks, it might be because insurance won't cover it, so be ready to pay out of pocket. Or find another doctor who does. You need an ultrasound in your sixth or seventh week after the first day of your last period. After that, they are not as reliable to pinpoint when conception was.
When at the doctor, the nurse will always ask when your last period was. Just tell her that you are not sure when it came. (This is because you want to be sure they use only the embryo's ultrasound measurements to figure out its age.)
Anyway, get the ultrasound, and be sure they don't just tell you "weeks pregnant," you need an estimated due date. Also, if the doctor pulls out a little cardboard wheel and asks when your last period was, tell him you prefer to get an eestimated due date from the ultrasound. When you get the estimated due date, go home and either use an online conception calculator, or count back 266 days on a calendar, and that will be your estimated date of conception. For your sake, I hope it is not May 16-22, because then, you would need a DNA test, and if you want to do one before the baby comes, they are mondo expensive.