I slept with a girl on Sept 1st and I used a condom and she also said that she was on the shot. She is not my girlfriend, Unfortunately it was a one night stand. I have worked with her for a few months and recently she has decided to go back to school. She called me three weeks after we slept together and told me that she was 3 weeks pregnant. Was doughtful that I am the father. She did go to her first ultra sound and the doctor did advise her that she was 5 weeks pregnant (2 weeks after her period and three weeks after she slept with me). So I understand now that in fact it is possible. I have a good career and am happy in life, It is very difficult to talk to anyone about this, family, friends or colleagues.
She also stated that she has not slept with anyone after her last period, she did say that she slept with someone before her last period which was before August 18th (roughly). She is 100% sure that it is mine.
The most difficult part is the fact that she is only mentions financial support and seeing a lawyer. I must admit I am very defensive with her and in turn she becomes defensive with me.
I just don't know what to do at this point. I know I have to wait and see.

It could be your baby, but then again it may not be. Just because she says you are the only one she slept with doesn't mean that you are. My best advice would be to get your own attorney, wait until the baby is born, then get a DNA test. Most women don't get a period while on the shot... some do, but many don't. All you can do is wait it out. Hopefully you can be on more civil terms with her, but I understand why you would feel defensive toward her. Talk with your family. Let them know the situation you are in. They may be more understanding than you think. Definitely talk to an attorney. They can tell you if you have any obligations at this point, and can also help you to get a court order for a DNA test after the baby is born. Unfortunately you can't find out for sure until then. Good luck to you, and hopefully everything works out for the best.

So it does sound like the possibility of the baby being mine are quite likely. Just wondering if the doctors assumption of how far along she is; Is it possible that she could have got pregnet before her period (august 18th roughly) and the fertilization (not sure if this is the right word) took place around that time. Does that make any sense?

If she is lying about having slept with other people it is questionable if she was on the shot or just saying that also. And I had a period with my first child until I was three months and all the doctor goes by are the dates of her period and the size of the baby during ultrasound and as most women who have had babies can tell you all babies mature at their own pace so the due date is a educated guess at best. I suggest you get a lawyer and get a paternity test as soon as the baby is born. Don't take anyone's word for it get the test to make sure. Good luck