I've recently started riding a horse for a friend of mine (who doesn't ride her very often) and who has just moved away so is a bit preoccupied. I've ridden Jaz before and she was always fine, but last week I rode her twice and both times she behaved very strangely. She would be very reluctant to move up to trot and when she did she would flick her tail and throw her head up and jog about so that I kept losing my seat.
She'd be fine in canter and walk. I wondered if maybe she was lame (to my knowledge, I've never ridden a lame horse, so I wouldn't be certain) but I lunged her and she didn't seem bothered at all in trot. I'm wondering if her saddle is perhaps pinching her. She's put on a lot of weight recently and I imagine it would be worse with a rider in the saddle but I'm not sure.
Does anyone have any idea what the problem could potentially be, or has experienced similar problems with their horse?
She also coughed a few times when we moved up into trot, if that's any help.
Thanks for reading and I'd really appreciate any ideas.