4 year old daughter suddenly crying at nursery school?

Ok...sorry I think this will be a bit long! So... the little lady started at nursery at the same school her big brother age 9 attends. She started in September, on a full day basis after going to stay and play half day until that point. She was used to being dropped off and was always looking forward to going.

She loved everything, right down to swimming which they are lucky enough to do as they have a small pool attached to the school building. Monday week, after the half term break, she was tearful when she got to school. Unusual for her, but she was ok. Tuesday, same thing. Wednesday (swimming day) cried a little when I left, then refused to get in the pool. Cried until she was re-dressed and returned to class. Last Thursday, bit of a wobble, but Friday was fine. This week: AWFUL. Monday she screamed, Tuesday she cried for two solid hours. Yesterday refused to go swimming AGAIN, but was offered in the afternoon (they have morning and afternoon swimming classes) and she went, saying she had a great time. Today...I had to put my foot down and I left her screaming again. Now the head of early years wants a word (like its my fault) as bubs told her she doesn t like school.

Head of early years has been off for past 4 weeks and is a bit of a dragon having changed since becoming head of early years.