I'm thinking it's either medical, or that she's just being pissy and cranky. At the moment I'm having the same issue with my usually-a-snuggly-teddy-bear Loki. Half the time he refuses to be caught, he's irritated most of the time, and just generally on edge. Under saddle he's normally stubborn and ready-to-go, now he's slow and pissy. With him I think it's because;
A) He's protective of my mare, and Divo's new AND an ex-stud
B) The weather has been really funky
The weather has been either freakishly hot, or weirdly cool, in my area and a lot of horses have been acting weird. Normally calm horses are high strung, normally docile horses act aggressive, easy-to-catchers bolt the moment they see a halter. Cuddly addicts don't want to be touched, etc. It's like everyone's on a hair trigger just waiting to freak out. So far no tornado's (really, really, strange) but I think theres going to be a nasty, dangerous storm coming and they can just sense it.
I live in Missouri, over by Kansas. I'm going to sound like a quack, but the weather is not going to be pretty here pretty soon and the horses here know it. Theres literally nothing wrong with ANY of them (this isn't just my guy, but several horses) but everyones acting strange.
Wait! I'll fix it....