Can you clarify the circumstances here, namely:
- does she work in your bar? If not, how similar is the place she works (same city? Same type of neighborhood? Same type of services?)
- did you work the same hours / days / crowd demographic? (There may be a difference in tipping expectations on Wednesday mid-afternoon when people are getting a single drink after work, vs Friday night when they're getting multiple drinks and food and trying to impress their date.)
- were there significant changes in the surrounding area, such as a factory opening or closing, that changed customer demographics?
- are there other wait staff you can compare to that would show a pattern? For example, are women in your establishment generally better tipped than men?