When she comes home, I would talk to her and ask her what happened today, why she was upset about being at school, did something scare her, etc. It may be that she had an argument with another child yesterday, that she just realized that this is going to be an every day kind of thing, maybe she's getting a cold, or didn't sleep well, it could be any number of things. If she doesn't have a reason for why she was scared, or if she is now having trouble seperating from you, I'd recommend one of the following to help her adjust:
- give her a little something to hold for you during the day that she can give back to you at the end of the day.
- give her a little picture of you that she can keep in her pocket if she misses you.
- set up some special goodbye routine for when you drop her off and a similar routine for when you pick her up.
It is a little unusual that she would have started crying again, but not totally unheard of. Watch and see if it continues on other days. If it does, and you try the things above, you will have to decide whether to leave her in or pull her out. Some kids really are just not ready for a school setting at 4, others just need time to settle in. If you leave her in, she will eventually settle down and stop crying, the only concern would be if it is leading her to a negative opinion of schooling. If you take her out, you can give her the same academics at home as she would get at preschool, and then join a playgroup to ensure she gets a chance to interact with other kids. Either way, I'd try to make sure she makes some local friends her age this year so that when she goes to kindergarten she'll have some friends there that she already knows.
There are mixed opinions generally on what is the best way to handle these things, but honestly I'd just stay with her. In fact I'd recommend that you don't wait for her to start crying at all, but that you just go ahead and let her know that if she needs you to stay that you will stay until she's ready for you to go. If she is never ready, then about a half hour to an hour before the end of the day, tell her that school is almost over, but you need to run to the store really quick, and get something, and then you'll be back in an hour, right about the time school gets out. At the end of the day talk about how it wasn't much difference having you there or not. Try just dropping her off the next day, but if she is insistent then stay with her, but this time for less time than the day before. Continue doing this until she gets to the point that she's comfortable.
Also, make a point of taking some special time when she gets home from school to do some fun "big girl" activity with her so she's still getting lots of one on one attention from you. If she knows she'll have special time with you when school gets out, it'll make it easier for her to get used to the time away.
She's probably just had the novelty of the experience wear off, and that's why you are seeing the sudden change.