I wouldn't agree that the first incident is a red flag. People use the sick excuse now and then we all do it. Itwould have been a red flag if she said to you "I always use the sick excuse" or you started to see it happen more than is normal.
She did take off more than is normal. Also, the first impression that the principal and everyone in the department had of her was not a good one. In a few months, she missed 10 days, which is a lot for a teacher. Also, all student teachers know that you do what your mentor teacher asks you to do, no questions asked. She also used this excuse when canceling plans with me, and she once told me that she's good at canceling plans.