Same here as Fleecewife - leave it be but keep a close eye. Sometimes the lamb starts to drink from both sides as it grows and needs more milk, but a mule probably has more than enough for one on one side, in which case the larger side will eventually subside.
If you milk it, you are creating demand, and it won't subside... but if you suspect mastitis then of course you must milk it out, several times a day if poss.
As she's a pet, hopefully you can catch her and have a feel if you are worried at any time. It should feel soft, warm and pliable. Hot, sore, hard, inflamed-looking - all would be warning signs of incipient or actual mastitis.