Hi emmypaige- I haven`t posted on here for a while but while visiting and spotted your post, I wanted to say I agree with Serendipity regards having this lump checked by a vet, preferably one who won`t have a feel and send you away with a guess or antibiotics if they `think` it`s an abscess, which would usually need aspired or drained to determine if it was an abscess or a small growth. Robos are very skittish little things and I know it`s stressful enough just to get them to a vet! But my last Robo was well into his second year of life when he formed a lump on his shoulder and it grew bigger and he lost a lot of weight.
These things can be removed by surgery but the anaesthesia can be the biggest danger. At 18 months he`s not young, but Robos can live a long time beyond this if they remain healthy and lump free. I truly sympathise because I`ve lost many small rodents to lumps of one kind or another and it`s soul destroying.
Many vets will recommend leaving lumps alone due to age and the lifespan of a hamster, but I would honestly say if the vet you see is more positive that removal would be best and your little man would be home the same day resting and then racing about again within a day or two, I would say that`s a risk worth taking. But the vet is the one who will give you the options. Stay positive and post back.