mare udders filled :S help!

My mare, who is definitely not pregnant by the way, has recently shown teats which look almost bagged up! Whatever fluid is filling the teats, it cannot be expressed, and I hot towelled her tonight. She isn't sore, so it isn't likely to be mastitis...
I just want to know why this may be happening, and what to do about it. I'm going to call out the vet just to have a look but just wondering what this could be. I don't know of her breeding history, so she may have foaled before, and someone has suggested that when in season some mares that have foaled can produce milk,...
Please, any info would be appreciated, or advice

I can "milk" my mare on a daily basis, and during her heat cycle, her udder is bigger than usual. But you say nothing is coming out? How big are her udders?

She may be having a false pregnancy as well.

If your mare foaled before during any heat cycles her udders ill fill up like she is pregnant.

My mini mare did this, only she was able to express milky fluids and there had been a possibility she was prego but I had the vet out and he said it was simply the lush grass that was messing with her hormones and with it being that time of year maybe the same thing is happening with your mare? It's puzzling that she can't express the fluids tho, maybe bug bites make it seem swollen? I know that also has happened to my mini mare

Hmmm.. thanks guys. They're not a huge amount bigger than usual, and they've been going down now thankfully. I'm not actually sure I know how to express milk properly to be honest, how do you do this?

My mare always has "milk", I just pinch up at the top and pull down, letting the nipple slide between my fingers and thumb, and I get milk,