I need a bit of advice.

[The sad one.]

I have a small shiatsu and the other day I found a lump underneath his left arm, I felt it and I am scared thinking "What if it's a tumor?" It scares me because this dog is the only thing that I have that's closest to a friend. I think if it's a tumor it's a mast cell tumor and I've reseached it. I've taken him to the vetenarian and they say it's nothing. I'm still worried though...

[Verità]

Try taking him to a different vet perhaps? Get a second opinion if you are seriously concerned. And even if it is "nothing", have them explain to you exactly what it is, to ease your mind.

[Rossiu]

Hey, I actually work at a dog kennel myself, with a vet attached to us in the same building and it is not something that I would be worried about if I saw it.

If I saw it on a dog in my (I say my even though im an employee) facility I would take note of it and check on it once a day until they left making sure to tell the owner and make sure that they are aware of it.

I would only be alarmed if it started to change size or change at all for that matter.

I had a dachshund in my facility that had such a lump and when I noticed that it had grown we took him in to the vet and as it turns out his arm was caught in a closing door a week before he had arrived and it simply needed to be drained.

If i was in your situation I was check on it once a week and if it never changes then I would not be worried.