How can I be sure that the child is mine?

Keep in mind that a U/S is just an estimate of the due date. They can vary greatly. My first two children were born right about their due date based on my last period, but the U/S had moved the due date up both times by 2 weeks. The only thing you can really do at this point is wait and when the baby is born, if you are still concerned, do a genetic test. You can buy them OTC now at some pharmacies and you just send off samples from the child and yourself. She wouldn't even have to know if you so wished.