We have a young mare that was one of the nicest fillies to train. This fall she changed and the only thing I can pinpoint around that time was that I noticed she was in a super strong heat (more than I've ever seen her act). She hasn't been the same horse since. She acts more worried when you are riding her, looks back, will be spooky at random times and then scare herself more. I have ridden a lot of mares and never had one act quite like this and non of my mares have been on any mare supplements or regumate. We've had her teeth done and chiropractic maintenance a few times. Nothing concerning there. I can get her ultrasounded to see if she has an enlarged ovary, but wondering if anyone has experienced something like this with such a change in demeanor. Thanks!

Sure have.

It was such a drastic mood difference I suspected a GC Tumor. I had my mare palpated, ultrasounded and a particular blood test run by UC Davis that shows multiple hormone levels. She palpated showing a retained follicle but her blood tests were within the normal range. We treated for ulcers just in case also.

Thank you! How did they treat the retained follicle?

The didn't.

I had gotten hurt and was not going to be riding so they let nature fix it.

I would first take a look at what you feed. 

I have seen (and experienced) many mares/horses have these symptoms when fed alfalfa and/or soy. I don't know the actual cause for this reaction but many horses go through it. Balancing the calcium in the diet with phosphorus and magnesium helps but eliminating the alfalfa and soy has a direct effect on the endocrine system. 

This is just what I have experienced.

Happened to mine!!! Regumate 365 days a year fixed hers. SHe cannot even stop in winter time. Vet says southern CA is not cold enough to keep her out of heat. 

hmmm she is on alfalfa/grass round bale and ADM primeglo feed. None of that has changed in the three years we've owned her. 

Well we get plenty cold here. LOL! I am in MN. So, she likely hasn't been cycling all winter. I've tried to watch if it more often when she's in heat or after in heat, but with 15 head it's hard to keep track. Some weeks she's better than others for sure.