It may be the same that happens with one of my retics: she'll not rub at all for weeks/months then she'll get a random stick up her butt and rub so bad that her lip will swell and I'll need to swab it daily with diluted Betadine until is goes down then she'll be good for several more weeks/months until it starts over. Upgrading cage size did help though: maybe you can put her in a larger tub/cage and see if that helps any?