My mare has swellings on her tummy! Help!

I have two mares which I have owned for 13 years. Jess is a 18 y o Welsh cross and Faeroe is a 16 y o Arab. The problem is that they both have swellings from their belly button to her nipples. One long lump which is central and right next to her belly button and one either side slighter nearer her nipples. The lumps are raised almost a centimetre. They are on good grass and have recently put on weight. I have been monitoring them and I am happy for them to put on some weight as they got thin in the winter. When I first noticed Jess' tummy, I was really worried, but the lumps are soft and not tender and it is strange for both mares to be affected. Faeroe can be a bit sensitive on her tummy, but both mares are okay if I press the skin and put on fly repellent.
Someone has suggested an adverse reaction to the flies? What do you think? Has anyone experienced the same thing? As I say I have had both mares for 13 years and never known such large swellings. I have been looking to see if there are any new weeds in the field which I had not noticed - which cause this reaction.

Does it look like a swollen vein? Maybe a lot thicker?

I think they are just fly bites, moo had some, put a fly rug on and she is fine :)

The lumps are quite uniform, possibly like a swollen vein. I shall take a pic tomorrow for you to inspect, then you can see what you think.
Obviously, mares (and probably geldings) do tend to get bitten a lot more around their delicate tummy area, but they are really puffy. Very odd, but I shall try putting my Bo-ett back on my mare and see if it makes any difference.

There is a gelding at my barn who got what looked like a VERY swollen vein under his stomach, the vet said it was because of the season change (it was starting to get hotter out) and that it might not change until the fall... but it's gone down. Idk if that helps at all... if you put up a picture I could tell you if that's what it looked like.

Thanks for your advice, that does definitely sound like a possibly. I shall get back to you tomorrow, Thanks again, x

I would like to see a pic.....sounds like something we were just discussing tonight at the barn about two othe boarders. Barn manager said it was cause they were fatties! Two large long lumps on underside.

It could be fly bites. :) I'd also like to see a picture.

Hmm since its two mares I doubt this is the problem but I had a mare who had a huge swelling on her underside of her belly it turned out to be mastitus which was odd bc she wasnt breed in 15 years but anyway its a thought eventually we had to milk the puss out of here she was a good sport and was very patient!!

My mare got "milk veins" under her stomach from making too much milk when we weaned Kadie off of her. It wasn't mastitis, and went away after a couple weeks, although I don't know what it could be for your babies if they haven't had foals in a while. Would love to know when you figure it out, and good luck!