Small nodule on dog's armpit, what is this?

I have a 6 year old Black Staffordshire Bull Terrier. He recently had a small pimple under his armpit. A somewhat greenish colour but still mostly fleshy and shaped like a tomato pip moving directly up and out. It stood up quite tall for just a normal spot. Underneath is a bit of red swelling. The pip shaped pimple came off and the swelling is going down a bit but it looks quite sore. There doesn't appear to be any puss yet but I'm just wondering what it is. I've looked all over the internet and I just can't find a match.

Could have just been a pimple or a cyst. Please go to the vets to have it checked out

My dog had one of these and we took it to the vets, he said that it was a fatty lump and as it was quite large it would be best to have it removed and analysed. We did and it didn't turn out to be anything but just what he said a fatty lump, however the operation was quite a big one and expensive, and now he has another one.. I would say get it checked out but if it is harmless , leave it as its a lot to put a dog through if it isn't serious