It sounds like there is a good chance that you are not the biological father. The only way you could be is if the ultrasound was way off (the one that said she was at 20 weeks GA). Ultrasounds *can* be way off by the time the pregnancy is in its 40th week, and can even be a couple of weeks off at the 20th week, but probably would not be far enough off at 20 weeks to fit the date of you and she having sex.
Before you say anything to her, though, consider first that in such a situation, some men are OK with continuing in the father role whether or not the woman got pregnant before they met. Meaning, they love the woman and the child, so it is a matter of indifference to them that she was already pregnant. They don't mind knowing for sure, but don't intend to walk away from the baby or the relationship even so. Another possibility that you should keep in mind is that sometimes the woman does not know she was pregnant when she gets together with a new person. So it can take tact to talk this over, certainly you have to handle it without accusing her of fraud. Think over what you want to do, before doing or saying anything.
If things aren't close between you and her and you decide you don't want to continue being called the father if you don't think you are, you need to do a DNA test at a reliable lab (no drugstore tests) to know for sure.i You need to arrange a legally admissible paternity test, done at a reliable lab. The family courts in your area of jurisdiction can tell you where to go for the test.
If you cannot get the child's mother to accept that you want to do this test, see an attorney. You should not be forced to support a child if the child is not from you, and the attorney can tell you how to go about disentangling yourself from any future claims she might make. Don't wait either, since if the court were to put in an order for child support and you were not to pay (and didn't challenge it legally) you could owe some money.
In short, if you are only being supportive because you think the child is from you, but would not be supportive or connected to the child and mother otherwise, you need to move on proving the genetic connection between you and the child or lack thereof. Get moving, if you want to be out from under the obligation.