I have no idea what it could be. I can tell you what I would do....straight to the Emergency Vet. It would be worth the $150 charge to walk in the door to have them tell me it was nothing....because if it were something, I would not want to have it get worse during the night.
The thing that is a concern, is that her personality has changed. It could be nothing, but to me better safe than sorry.
Good luck and welcome to YT!
Please keep us posted on her!