If it was an ordinary period, that looked, was timed and lasted as long as you would expect for your period, it sounds like your boyfriend is the dad and not the guy from the sex in early December. The December guy's sperm would not last that long or through a period. (I assume that the guy from before your period is the person you've called Guy B, even though that sex was earlier than Guy A? And that the one after your period, Guy A, is your boyfriend.)
Anyway, to check for sure, if your last period was December 23, go to an ob/gyn now and get an ultrasound. Counting from your first day of your last period, you would be considered to be at about 6 weeks 5 days "pregnant" now (doctors begin the count on the first day of the last period), or about 4 weeks 5 days since conception, if that helps.
Because women can ovulate at irregular times, counting forward from the last period isn't always accurate for pinpointing ovulation. If you would like to be ultra sure the doctor doesn't just use your period date to compute how far along you are (and would like to double- or triple-check that it really was a period), tell him your cycles are irregular or that you forget when your last period was, and that you would like him to compute your due date just from the ultrasound data.
When you get an estimated due date from an ultrasound without it being influenced by the first day of your last period, take that e.d.d. home and put it into a conception calculator online, or just count back manually on a calendar 266 days. That would be your estimated conception date.
You said that you ovulated on a certain date. Were you using ovulation test strips at the time, so you know this? Or was this just from an app?