You need to look at the importance of things right now: she may have been a bad friend and rude to you, but she is going through a very hard time right now and it may be best if you two just talk things through rationally. Getting angry and yelling at her just for the sake of a little revenge is not going to help either of you out in the long run, but helping her bring a nice baby into the world and if you two can salvage your friendship, that is something that will help you in the long run. Have a girl's night.. order a pizza, rent a movie, and just talk through some things. Tell her that you heard her laughing about hooking up with this guy, and ask her if she did it to intentionally hurt you. Tell her that it is important for her to be honest with you. Ask her what her motives were and why she would want to hurt someone who she considered a friend. I hate to say this, but some girls, especially teenagers, can just get an evil idea into their head and it makes them happy to have that self fulfillment that they get by hooking up with guys. If that is the case for your friend, then I feel very bad for her and her situation with the baby. Hopefully she'll get a better sense of what is important in her life after the child is born.
If she says she got together with this guy you liked to hurt you, then that is horrible, but if there is any way of you two working together through it, you will most likely be much better friends because of it. It'll show both of you that you can talk honestly to each other and get through things. That is what friend/relationships are about. Also, what is your source of information? It could be a very dirty trick of another person to plant such a rumor and hoping it'll tear you two apart. Talk about it.
I see no problem with confronting her even though things are going on in her life, but make sure you do it in such a way (such as a girl's night) that doesn't put either of you on the defensive right away. If things get too heated, you can always just agree to talk later and watch the movie. (Whether you would choose to actually talk later would be something you would have to decide if that situation arose.)
It sounds as though you really do care about your friend, her feelings, and her life. She's lucky to have a friend like you who wants to sort things out even when they've been hurt. If you want any more info or I can help you in any other way, send me a message.
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"If you stand in front of a mirror holiding a dozen roses, you'll see
thirteen of the most beautiful things."
"Don't have regrets because at one point, you were doing exactly what you
wanted to."