What does "I took urine tests every day up to 2 weeks and 13 days after sex which all tested negative" mean? Two weeks plus 13 days? Two weeks *or* 13 days? Just 13 days?
In trying to sort this out with only the information you have at present, the problem you face is that sperm can live up to 5 or 6 days in your body after sex. If you happened to ovulate on January 4, the sex from December 30 could still produce a pregnancy. For this to happen, 'pulling out' (in other words, his not ejaculating) would have to have failed, and some sperm travel into your body on his pre-ejaculatory fluid. (Possibly this sounds like an unlikely event, but in fact, pulling out to try to prevent pregnancy often fails. If a guy had recently ejaculated before having sex, he can have lots of sperm in his urethra, and it hops onto the bus of pre-ejaculate and rides right in to the woman's body. Women write into this site all the time who got pregnant by a partner that did not ejaculate in their body.)
If you ovulated on January 4, probably about the very first day in a best case that you would get a positive on a blood pregnancy test would be (at the earliest) about 8 or 9 days later. Though this isn't guaranteed -- the embryo can float around up to 12 days after ovuation before it settles into the uterine sidewall and begins to put out the signals that a pregnancy test picks up.
So, your doctors are right. The info you have at this moment is not enough to (for example) rule out the first guy.
This leaves you some options to try to determine who is the dad. I've posted the following list in this community before but here it is again. (You have done the first thing and come up with the doctor saying it is too close to call by that method.)
The tests to determine who a dad is before the baby is born are, in order of time (when to do them):
1 - Take your calendar in of when you had sex and ask your doctor to help you understand when you conceived (when you ask, use the actual c-word "conceived," don't just take a number of weeks "along" as an answer to this question). Explain when you had sex with whom, and see if the doctor can advise you as to who is the dad by the dates of the sex. This is the cheapest and easiest way to get an answer. If the dates are too close to call, the doctor will say so.
2 - Get an early ultrasound, for example around five weeks after you got your first positive pregnancy test, and have the doctor give you an estimated due date from the measurements of the embryo. (Not a "weeks pregnant" count, but a calendar date.) Then take that due date home and either count back 266 days from it on a calendar, or put it into an online conception calculator, to get your estimated date of conception.
3 - Get a prenatal DNA test. Don't use one of the cheapie so-called labs, either use Ravgen or the DDC. They are costly, so only go this route if:
- you have the big chunk of change that they cost, and
- you are willing to test with both potential fathers. (One man's positive result will back up the other man's negative result, serving to provide you peace of mind.)
Neither Ravgen nor the DDC's tests are invasive nor harm the baby; there is a blood draw from the mother (her arm) and cheek swabs from the potential fathers. There are many charlatans in this field, so stick with one of those two labs. We see them mentioned favorably often on this site and the only times they have been bad-mouthed, it seemed to have come from a troll.
4 - After the baby is born, get a DNA test for paternity with both fathers. (It is much cheaper once the baby is born than the prenatal tests are.) Use a legitimate lab recommended by the family courts in your area of jurisdiction, and go in with the guys so you can affirm that they haven't sent their buddy in their place.
If either of the possible fathers will not willingly test before the baby comes, you will need legal counsel to enforce a test after the baby comes. Don't test with one guy and just assume the other guy is the dad if the first one got a negative result. You need to be sure from a certified lab where people go with I.D. and are photographed, that you have a legitimate result.