I agree with the other posts as a most likely cause, but did had two other thoughts as well. The first is possible parasites. I know it sounds weird, but I had a mare pick them up (we think from a stall at a show we attended) I couldn't figure out why she the entire winter so darn cranky and jumpy. she didn't do anything bad like kick, just acted like she didn't want to be messed with and seemed super sensitive about everything. I was horrified when nice weather came and we clipped her. It looked like a big grey cloud moving across her skin. I took one to the vet and he said he wasn't really sure what it was as it didn't really look like a louse. All I know is I felt horrible she'd been that way for a couple months. She didn't drop weight, didn't have patchy hair spots etc., just aggitated. My second and more likely thought though was neighborhood kids. If it isn't her cycles causing the problems, I would check for the bugs, but I would also keep an eye on the pasture. My farrier brought his son out once and when I went to the other lot to bring in a couple more, I came back to find the kid chasing my two foals around the lot so hard that one of them actually ran into the fence. Needless to say I was highly ticked. The colt was ok, but after that the filly was always a bit leery of being caught. So it might be possible some kids (or weird adults for that matter) could be messing around. Hope whatever it is you are able to figure it out and that things get better.