Looks like it could be a bite of some sort.........my first thought was it could be edema but not if it is warm to the touch. But because it is so close to her udder I would have it looked at by the vet.
My gelding had some pretty nasty bug bites one year........probably 3 to 4 times bigger than your mare
That was my thought Super Nova. Either an allergic reaction or a bite. If it was edema from the udder in would be lower on the stomach and in front of the udder not in the stifle are that it seems to be swollen in the most.
OP let us know what you find out when/if you have the vet out.
Well the vet emailed me back and agreed with you guys. He said it looked like a kick or a bite and if it was still there by the weekend to call him. Well.... its gone! Sheesh these pregnant mares keep you on your toes!