I gave my horses their annual shots on Monday. On Wednesday I noticed that my mare had some swelling on her belly, right behind the girth. It's bigger than the spread of your hand. It wasn't hot, didn't seem painful to her, and she didn't have a temperature, so I thought I'd just keep an eye on it. Then this weekend she developed 2 more lumps futher back. They literally popped up over night. Again, they don't seem tender or hot. These two are about the size of a fist.

Her appetite seemed a bit low so I decided I better take her to the vet yesterday. But the vet was stumped too. She said they aren't soft enough to be something you could drain, but are too big to be bug bites. There are no puncture wounds associated with them or anything like that. She told me to just hose them with cold water for 10 min a day and if they are still there in a week, come back.

Has anyone ever seen anything like this before? Any ideas on what it could be? Maybe I need a second opinion?
The vet said she didn't think it was an allergic reaction since bumps are showing up so far (a week) after she had the shots.
Nothing has changed in the mare's environment or routine any time recently. My horses are on my own property so I'm very certain about what they get to eat, where they go, what they are around. No one else has access to them.

Lusitano--I'm in Wyoming so that kills the bug theory

I have a gelding at my barn that gets those all the time when i worm him.It must be something in the shots and wormers.She should be ok though.

Has your barn changed the type of hay they are feeding or the place they get their hay from? Or have they changed the type of bedding? Have you changed fly sprays or other grooming products? Your horse could be having an allergic reaction to something besides the shot. She could be sensitive to something in the feed, bedding, products etc which is causing the lumps. I have seen that often in horses at the barn I used to be at. Several would break out in large bumps after we got a new shipment of bedding in. If your barn hasn't changed anything, I would guess it's just a reaction to the shots. If they have, ask your vet about possible antihistamines to keep on hand for the next time the barn makes changes. Don't be afraid to ask your barn manager about any changes. Remember, they are there to take care of you and your horse and should be happy to let you know.

Well, do as your vet says and if they don't clear up in a reasonable amount of time, call her again.

There are no bugs in Wyoming?

I had this prob before in NH. Barn spiders is one answer and the other is a reaction to oil based fly spray.
Yup...I got hit with both of these.

The barn spiders would climb on him when he was laying down and bite him.
One even bit a neighbor kid and his eye swelled shut and the skin turned black. Nasty
The other time was when I used what is called "Horse and Pony Spray" which you can find at TSC. My gelding got the huge swellings all over his body. He doesn't get it if I use water based sprays.
A bath in Betadine shampoo got rid of the oils.

Have you thought of maybe something in the shavings? Maybe the shavings are not 'clean' and have treated wood in them.

Hope you find out what it is! I know you must be going nuts!