Well if you got your period then it's probably not guy number 1, i think it's probably your Boyfriend, cheers! :) <3
IF you had a "normal" period in December then it was unlikely you were pregnant then. It is VERY unlikely that you would NOT get pregnant having sex 2 days before your period. You are just about at your lowest fertility, the egg was likely NOT viable.
If you were only with your boyfriend after that incident then he is likely the father. Sperm can, at best, only survive about 5 days.
I doubt you ovulated on January 18. That would be very late. More likely it was between January 7 and 12th. It normally happens between days 11 and 16 of your cycle. Day 1 is when your period starts.
No test, including an ultrasound, can tell you exactly when you conceived. It's all an approximation. Fetuses, like children, all develop at different rates.
Again, more likely your boyfriend is the father. A DNA test could be used to confirm it but if it was truly a "normal" period, then it is unlikely you were pregnant then.
Hope it helps.