Oh the poor darling
My passed roborovski Cookie had something that sounds very similar to your description and was also under her left armpit, it was indeed a tumour. It went from being flat to being a lump within a few weeks, the vet drained it twice and she was put onto antibiotics for a few weeks but it kept growing bigger so in the end as the vet trips were too stressful we just let her be and she lived for about three months with it before she became too weak and we made the decision to have her PTS. I have some photos of the tumour here that I hope will help you out, I will warn people that these photos aren't very nice at all and if you are easily distressed please do not look at the third photo.
This is her tumour at the beginning, similar to how you have described however this was after it had started to become a lump -
This was it about 2 months later -
And this was it just before her passing -
I hope these photos help you out and that the little one will be ok, like I say the black patch/lump was the very begining of the tumour and she lived for three months more after this so if the hammie does turn out to have this and nothing can be done she may still have a long time left with you. Good luck at the vets!