Your mare could have mastitis, mine got this few weeks ago, just after being in season- get your vet to have alook she may need some anti-biotics. If left untreated mastitis could make her very ill.

or like you say could be a bite thats just gone on a sensitive part of her xx

I have had alot of problems with my mare being in season and i have had her 6 years, so there are alot of things and body changes i have seen of her. Her udder often swells up and is full of milk,even when she is not in season but she is a very broody mare. BUT id perhaps be able to tell you more if you could tell me how old, if shes had a foal, and how regular/irregular her seasons are. Would be interesting to hear back from you, had 5 mares so have seen quite a few strange goings on but ya can never know enough so would love to know more x