I'm going to be brutally honest here, because you need to hear this: That baby is yours, almost without a doubt.
She started acting weird, because she knew she had to go bang someone responsible and trick him into believing she was having his baby.
I am speaking from my own experience and that of my many female friends -- it is very, very rare for women to carry on sexually repeatedly with multiple partners. If she was regularly having sex with you, she wasn't regularly having relations with someone else. You can take that to the bank.
If she was having regular relations with you, she stopped at the time she started having regular relations with someone else. If the baby was born very recently and was fully cooked, you can just about do the math of when conception occurred. Were you with her during that four or five day period? She probably WASN'T with someone else, and even if she was, if she had a longer sexual relationship with you than with him, it's STILL most likely yours. The only way it's less likely to be yours than new guy is if she was unable to have an orgasm with you. But other guy was able to rock her world.
Statistically, you're either marginal in the sack or that baby's yours. Congrats.
