My girlfriend's last LMP was 11/28/2011..admitted of sleeping with another guy on 11/29,11/30,12/1.
she slept with me on 12/10, she had lite bleeding on 12/21,12/22.
Her first ultrasound was 2/20/2012, dating her at 12 weeks exactly with an due date of 9/3/2012.
I guess my point was really that it is possible that it is your baby still given the dates, however there are other things impact these dates such as average cycle length, which is generally 28 days, but can be shorter or longer depending on the woman.
With only a day difference between her having sex with you and then with the other guy- I believe your only sure bet is going to be a paternity test.
Also, Sperm can live 2-3 days inside a woman before it dies, so you also have that component of uncertainty as well.
Sorry it couldn't be more of a clear cut answer for you.
Have faith in yourself and listen to your gut- that will help guide you in your next step!