I would go to the director of HR, have them pull a copy of your job requirements. If you are fulfilling your required shifts, I would file a formal complain against the manager and have her remove that bit from your evaluation. I agree with the others, she is trying to guilt you into picking up shifts. We all have our unique family/life stressors and challenges. I would look for a different job. I left my dream job because of a manager giving me extra duties, not in my job description, because she thought I wasn't busy (I'm just well organized). The extra duties were actually HER Duties.
Good Luck,