How to tell her..

How do i decline being a bridesmaid? This girl was my best friend in high school but once i left for college we drifted apart. Their wedding is next month and it seems very unorganized. She wants me to be a bridesmaid but has yet to tell me which dress i need to get. I will be 14 weeks pregnant when the wedding is and it is outside in the blazing heat. We dont really want to spend money on a dress when we need to be saving for baby things. Also i live three hours away from where the wedding is and it is on a sunday night. I will have to leave very late to get back before monday which is the first day of college classes. I just dont know how to tell her without being rude.