As much as I'd like to jump on your side here, don't get mad, I kind of want to shift you your ex-best friend's view. Did you, like Digital Brew said, talk to her before freaking out on her? I know, KNOW, jealousy can get the bad out of people, and not to point fingers but, If you didn't sit your EX-bestfriend down and talk to her about it, how was she to know? Many can't read minds, and if she really is pregnant loosing her BFF in a time of need would be an incredible stress causer, especially when she doesn't know the reason why.
However if you did tell her, and she went behind your back, I'd say talk to her first before calling her a bi tc h. People tend to make stupid mistakes and you never know, the guy could have made a move on her first or taken advantage of her. After all pregnancy is no joke here peoples, but again, if she's harsh or simply a liar, drop her like a baggage and don't look back.