Horse (mare) has reoccurring problem?

my mare has experienced a swollen teat (left side) now twice in the last six months, that's how long I have had her! it comes as fast as it goes away! it lasts several days and is gone! the teat gets really swollen and hard! It does not seem to bother her, she has no fever, not off her feed, I can touch it and she doesn't flinch and she moves normal! any ideas what might be causing this! she is not pregnant, and is 16 years old! I thought the first time it was a bug bite but twice now? just wondering if anyone else has a mare that this happens to? and what might be the cause! Oh yeah she had been breed before but the last foal was four years ago!