HI, I'm Dr STeve.

I am assuming that your horse is not bred.

There are things that can cause swelling in the udder. The most cmmon is when a mare produces milk due to being pregnant an an upcoming delivery. If she has been exppsed to a stallion in the last 10-11 months, she should have a pregnancy exam done to see if this is the problem.

Another possibility is an infection in the mammary gland ( mastitis) this is more common with a mare that is pregnant or nursing a baby, but it is possible that a mastitis could occur without a pregnancy, it is just not as common.

The third possibility is that due to changes in her heat cycle and the hormonal changes, she could have a transient ( short term) swelling due to the hormones in her system. Also some mares can have an imbalance of hormones that can cause similar issues.

All of these possibilities can cbe checked out by your vet by checking for pregnancy, checking the udder for infection and doing blood work to determine hormone levels . Once the exact cause has been determined, the proper treatment canbe started and she can get back to normla.

I hope this has helped, If it has would you please click the accept button so I may receive credit for my answer. Thank you and good luck. I will check later to see if you have more questions, please continue