Hi Tumi,
I have just seen this thread and although I haven't been in this situation one of my friends has and it was so hard for her.
I'm sorry to be the one to say this but can I ask why your boyfriend thought it was okay to go and sleep with someone else in the first place? I know I could'nt trust my boyfriend if he had done this to me, there is no excuse for it and I would'nt bank on him staying around in the future if that's his attitude! But if you have both managed to talk things through and sort it out then good for you
Back to the girl she sounds like a horrible person, do you know her personally or just because of what happened with your boyfriend? Does she know you are pregnant? If so my bet is that she is just trying to put the pressure on you to try and ruin your relationship and your happiness DO NOT LET HER!
Worrying won't get you anywhere it will not change anything, if you and your boyfriend truly love each other then you will find a way to cope with this, just think yes that baby is a result of betrayal but in the end if she and your boyfriend can come to a decent agreement with money and visitation then it could end up being a good thing your LO will have a brother or sister (whether you like it or not) who they will be able to take trips out with, play with and grow with! You just have to try and make the best out of a bad situation.
But I doubt it will work out happily ever after she will probably try to keep tormenting you and your boyfriend with this, get a DNA test done when you can and you will know once and for all at the most you should have to wait is 2 weeks until the baby is born! If she won't allow it, which is what I'm betting on then change your phone numbers and move on as best you can.
Congrats on your pregnancy by the way , get planning for your LO there are alot of things to buy and plenty different types to choose between