Can my subungual melanoma spread to the bottom of my foot I have a new small brown lump on bottom of foot looks like a small mole wasn't there before?

Yes it can . The answer is yes it can. It can spread to other areas of your body. But also, it is possible to be a second primary ( a second melanoma that is not a spread from the 1st melanoma). In any case, you should talk to your md/ dermatologist about this -have it checked and treated -asap.
You. Need to be under the care of a specialist. Melanoma is very serious.
Yes, it may. It is not feasible to know without examining the lesions, but if the primary subungual lesion is a melanoma, it is a malignant tumor and can spread any where in the body.