Nicole... So I assume you were on the 28 day pill such that you still ovulate around CD14 and get AF around CD28 is that right? Depending on how you were taking the pills and the type, you might expect AF during the last row of 7 pills and make the guess that ovulation occurred about 12-14 days prior. Did you stop on the 17th b/c you were done with a pack? Done with the placebos too, or just the hormone pills? Or did you just stop randomly? Where you were in the pack could help us pinpoint O for November.
If we assume that your bleeding was a true period, that would have put November's ovulation around November 12-14 and predict December's ovulation for 12/9 (based on averages, of course... every woman is different!). You mentioned DTD on the 14th, 19th, and 21st... of November? If you ovulated around the 9th of December, then none of those BDing sessions would be relevant to ovulation in December. So when did you DTD in December?
CD 1 11/26-29 bleeding
CD 14 12/9 possible ovulation
CD 28 12/23 expected AF
CD 31 12/26 HPT+ possibly 17 dpo
CD 32 12/27 B+ possibly 18 dpo (3.5 weeks pregnant as they say)
This is what your doctor is assuming and your hCG levels are around the median for this far along.
If you think that your bleeding was breakthrough bleeding or the rare implantation bleeding, not AF, and that you caught the egg in November... then O was not as assumed. Implantation occurs between 8 and 10 dpo 84% of the time. So if it were implantation bleeding we might assume this:
CD 10 11/14 DTD
CD 14 11/18 possible O
CD 15 11/19 DTD
CD 17 11/21 DTD
CD 22-25 11/26-29 possible implantation bleeding at 8-11 dpo
In this scenario, your BDing sessions of the 14th and the 19th could catch that egg if O occurred around the 18th. However, if this were true, your hCG levels are extremely low for how far along you would be. 99% of women would have high enough levels to test positive by 14 dpo. If you said you had an HPT- on 12/22 then this is highly unlikely. On 12/22 you would have been like 34 dpo and almost 6 weeks pregnant. While this can happen, it's not likely.
If you're thinking it's breakthrough bleeding around the time of expected AF, that shifts things slightly to even more unlikely.
CD 14 11/12 possible O
CD 16 11/14 DTD
CD 21 11/19 DTD
CD 23 11/21 DTD
CD 28-31 11/26-29 possible hormonal bleeding around expected AF 14-17 dpo
In this scenario, none of your BDing sessions line up with O, unless you had some earlier in the month. But this would also make you even farther along.
So with all this in mind, my best guess would be that your doctor is close... that you are indeed around 3-4 weeks pregnant. But I know I'll be very excited to hear the results of your u/s in about a month!