She will need to go to the vet, and the vet should do a biopsy of the lump. My pig Teddy has a lump under his nipple, and had to have a biopsy. Thankfully it wasn't cancerous, he was given some antibiotics as the lab results showed a slight infection in the lump. As it wasn't cancerous the vet said it wouldn't need to be removed and I was told to keep an eye on it and come back if it got bigger. It did get bigger so I took him back, but the vet was happy that it didn't need surgery. She said some pigs do get fatty lumps, and normally they don't need to be removed if it isnt impairing them in anyway, especially as she thought that it would likely to grow back anyway, and that some can shrink and go away on there own. The funny thing is over the last 2-3weeks the lump has shrunk on its own and has nearly disappeared! He still has a couple of smaller lumps (pea size) so I will keep an eye on them.
If you have a camera and a ruler/tape measure take a picture of the lump with the ruler next to it so you can keep a record of how big it is, if you think say a week later for example the lump has got bigger then take another picture of it with a ruler.
I can understand you are probably really worried, Iknow I was. But the chance of the lump being cancerous are very slim, and as long as the lump is not impairing her in anyway then she should be able to live with them without having any negative affect on her, there is always a chance too that they could just disappear on their own like Teddy's. Please dont blame yourself for only just finding it, Teddy's appeared almost over night, and again doubled in size within a week, I think they appear very quickly.
Hope this helped a little. Hope all goes well at the vets, we will keep fingers and paws crossed here x
