From this info alone, if the sex with the guy on the 21st was unprotected, you would have had viable sperm in your body from both guys at the same time, and that time period would include when you might have been expected to ovulate (judging from when your period came). Are your cycles super regular like clockwork and your new period comes every 28 days, month in and month out with no variation? Or are your menstrual months longer or shorter, or irregular?
The only thing you can do at this point is get an ultrasound, can you do that? From the ultrasound, they will give you an estimated due date for the baby based on its measurements and developmental markers, and from that estimated due date you can calculate when conception was likely to have been. But for this to be helpful, you need the ultrasound in the next two weeks, a later ultrasound won't help split the difference between the two guys (if it even helps now, which it might not. It might point to a date when both guys' sperm was alive in your body. But it's all you've got at this point.) In your shoes, I would definitely call your doctor today and try to get an ultrasound appointment in the next week or so, not a lot later. Do you think that is possible?
A little later in pregnancy, you could get a DNA test, but it's worth noting that prenatal DNA tests are extremely expensive compared to those done after the baby comes. If you want to go that route, call Ravgen or the DDC and get all the info, and call both guys to have them help with the price.
What is your relationship with each guy? Do they know about each other and that there's a chance they might not be the dad?