My mare normally has a lovely temperament but has recently been nasty and won’t let anyone up
Any ideas?

Could be pain related or just might need some time off to enjoy herself in a field:)
It could be pain related or heat. Some horses are nice during there time but other could be quite moody. My mare is a very calm and nice horse and is always nice when she’s in heat. She started getting all moody and didn’t like use riding her. We had a vet come out and look at her and she turned out to be pregnant. We didn’t think of that Bc it was winter so we thought she was gaining weight from us not riding her as much and eating a lot of hay. We also had our stallion gelded but he could still produce for like a couple months after. When we found out she was pregnant she was due in a month. We all felt terrible! So I would have a vet come out and look at her to make sure she isn’t pregnant. Also have a chiropractor come and look at her.
This is either an indication of pain as mentioned earlier or her hormones are all messed up. This can seemingly randomly happen to mares that are normally pretty good even during heat. It is usually due to weather changes. There are certain medications that can be given for mariness but I believe they are all long term treatments. Just talk with your vet.
Depending on what your mare is like during heat.. it could just be that time for her, though I've met horses that are still very nice when in heat.
It’s probably pain related. Potentially ulcers but I would have a vet or chiropractor out before any treatment is done.
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