She's obviously jealous of you. I mean, who jokes about starting a rumor and then insults someone in public? She definitely seems to be trying to "compete" with you or something, and she sounds like a real loser and a bully.
I'm sorry she decided to pick on you as you seem like a very kind person. Don't feel you have to give her or anyone else a reason for your weight gain--125 lbs is a beautiful and healthy weight for any woman. She's the one who needs to do the explaining for her rude behavior.
Please take care. I hope the rest of your work week is much better.