My first response has to be “what does your doctor think about it?”
I say that, because if it is a tophus, you’re in serious trouble.
The only way to prove it, is to get a rheumatologist to sample it. We can guess if it is or if it isn’t, but that’s largely a waste of time.
What I will say is that your description sounds exactly like the first appearance of gouty tophi in my experience. If it is a tophus, it will have started weeks, or months ago, but too small to notice. At some time it becomes noticeable. Of course, it could be something else, but if it is a tophus, it will continue to grow in size, and get harder. Eventually, it will cause serious discomfort, and probably burst the skin, risking infection.
The serious part is that visible tophi are just a signal that your body is riddled with uric acid crystals. In the joints they prevent natural repair, so bone, cartilage and tendons gradually crumble. You’ve noticed it in your largest organ, the skin, but uric acid crystals will spread to other organs, commonly causing damage to kidneys and heart valves.
Fortunately, this is completely avoidable by getting uric acid safe. I notice from your profile that you haven’t been tested for uric acid, so my advice is to get that done as soon as you can. Then please come back and share your results, and your doctor’s opinions. We can easily work out a plan to control your uric acid.
I had intended to make an appointment with a rheumatologist as soon as is practical – I work in 3 different countries and move continually, so “long-term” treatment can be a challenge. My earliest opportunity is when I return to Malta in 4 weeks.
Right now this lump looks more like a cyst/boil – so I am _hoping_ that the rheumy could remove most of the mass while sampling and planning further treatment.