I used the "Mama Natural" conception calculator with your due date of January 30, and it gave the date range for sex that produced this pregnancy as being May 2-8. Did you have sex with someone else then? If so, he has the likeliest chance to be the dad.
As as ppowb says, the time between the 8th and the 12th is not the widest of margins. This means you're not entirely free of the chance that the later sex produced the baby. But if you had a partner and plenty of unprotected sex during the period of the 2nd through the 8th, in your shoes I would (guardedly) assume the baby came from that.
You'll need to do a DNA test to put your worries to rest on this, unless you can tell by looking. (And I mean really tell, like if one guy is from Africa and the other is from Iceland.) If the fact that you had sex with the guy on May 12 is a secret from the earlier guy, tell the guy from May 2-8 that you need a DNA test to protect the baby's rights, and want it to be in the baby's medical record that a test was done. Do the test at the hospital when he visits, and go with him when he does his swab and watch while he gives it to the lab tech.
If you are married to the guy with whom you had sex on the 2-8, this won't in fact be necessary for legal reasons, because the law will assign paternity to your husband automatically. But if you aren't married to him it is definitely necessary to protect your child's rights to his or her father. And having taken a DNA test, you will also have your private worries about paternity handled by the test results.
Based on the first (most accurate) ultrasound your ovulation for this pregnancy was May 9th. Sex the preceding weekend is likely responsible, but May 12 is too close to be ruled out. Sorry you have been worryingly about this for so long.