Honestly - two things - first, I wouldn't take what she said personally. Maybe it was the way you asked her, maybe it was something she interpreted wrong, maybe someone else said something to her about her weight and she is feeling guilty - who knows?
Second - I would try again! Go talk to her again. Tell her that you feel like you may have offended her with your question and you didn't mean to imply that she needed to lose weight - you just thought that it might be something you would enjoy doing together.