I keep avoiding this girl and shes still trying to get a hold of me?

I've been avoiding this girl that use to like me lately and its because I don't want her to see how much weight I've gained. I know its stupid but I feel like she will trip out that I'm not skinny anymore. I asked my brother and he said he was being a realist with me when he told me I should avoid hanging out with her and try to lose weight before I see her. but she's wondering why I'm avoiding her I mean I could tell her the real reason but that shows a lack of confidence and that will be a big turn off. I feel like either way I'm screwd. If you could honostly be real with me and help me figure out what should do I would really appreciate it.