How reliable are the places where she has been getting her ultrasounds? Is she going to a private doctor or just a walk-in place? When she had the ultrasound either time, but especially on December 6, what estimated due date did the doctor give her? (Trying to figure this out based on merely a GA or "weeks pregnant" count is a road to nowhere.) Do you have copies of the ultrasound photos? A fourth-week pregnancy looks very different than a mid-fifth week pregnancy.
How regular are her cycles, does the period come every 28 days no matter what, just like clockwork? Or do they vary?
You are very specific and intent, and for some reason she was using OPKs (please don't tell me it was for birth-control purposes -- all they do is measure hormone surges, not the date of ovulation). Have you two been trying to get her pregnant? Is it causing an argument, that she had sex with her ex? You say darkly, "I feel like someone is not being honest." Are you two now on the outs or iffy?
Because the very simplest way to solve this (though not the cheapest, obviously) would be to contact Ravgen or the DDC and set up a prenatal paternity test for both you and the other guy to do with her in a few weeks. This would be more difficult if you two are arguing and you mentally accuse her of lying and she is responding by refusing to consider that you might be the dad. If you try to insist on testing in such a fraught emotional environment, there is the risk she will label you as obsessive or controlling, and simply refuse to test and refuse to try to get her ex to test. You would be able to get her to test in 9 months by going through legal channels, but it sounds like you are more than interested in knowing now.
I think from the data you give, you have a chance, not a huge one. The calendar does not give you much support (if she always has a period on the 28th day), the cramping that you interpret as ovulation helps your case, the OPK does not favor your argument, the fact that the ex says he didn't come means little (lots of times, there is still sperm in the urethra from a prior ejaculation, and besides, guys who don't want to be fathers always say this). Your assumption that sperm lives only 5 days is not necessarily supported by research, though the research is not definitive. (Some older research says 72 hours, some says 4-6 days, some says sperm lives 7 days but suggests it is not strong enough to penetrate the egg by the 6th day.) None of this can be used to disprove for sure that you could be the dad. The only thing you can do is ask for testing. If you have made her hostile or suspicious of your intensity on the question (or if you are doing this in a battle for control or to be right) then you are going to have to walk back the arguing part and be more constructive and helpful and supportive.
If she is willing to test before the baby comes, you and the other guy should both test. One guy's positive will back up the other guy's negative. Do not use an Internet cheapie so-called lab, call Ravgen or the DDC. Be ready to pony up the big bucks.
If this is not possible, talk to a lawyer and prepare to get a test when the baby is born.