Help! Itchy lump near hamster's ear, treatment not working?

[Opal]

Last night when I was checking on my 1-year-old dwarf hammy Eden, I noticed there was a bump near her right ear that I'm 99% sure wasn't there the night before. Wasn't sure what it was, whether it was some kind of tumour or abscess or anything. When I tried touching it it seemed solid but when I lightly squeezed the poor girl squeaked and jumped off my hand to her enclosure, so it was definitely uncomfortable/painful. There aren't any exotic vets in my city in Indonesia (that I know of), but I got into contact with a vet my mom and friend knew and she recommended I try topical treatment her with ointment gentamicin. I tried googling it and it seems websites are conflicted on whether or not gentamicin was safe for hamsters, but it also seemed like most of the websites were talking about the oral gentamicin and my friend told me the gentamicin worked on her hamster's ear infection, so I was hopeful it could help with Eden.

I got it and applied it on the area that was bulging tonight (pic of the bump from yesterday is below), but not 5 minutes later Eden began scratching herself more to the point of flipping over and trying to use the bedding or her sandbath to scratch it. I'm also worried the ointment might get into her mouth because whenever she scratches or grooms, she stops in between to lick the foot she's scratching with. Is it normal for her to react like this? Should I continue with the treatment? How can I minimize her scratching?

I'd really appreciate any assistance. Eden's had itchiness problems for quite some time now and her brother seems to be following suit, and I haven't figured out what is causing it. Eden's also gotten a few tiny bald spots under her belly but that's currently the least of my concerns. What should I do?