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It sounds like it could be a cyst...OR...and area that the dog is consistantly doing trauma to and getting a lump.
Tumors dont commonly come and go. I would recommend that if it is present now, a vet take a look.
If it is on the bridge or firm part of the nose..(top) it probably not affecting the breathing. If it is off alittle to the side or you see nasal discharge let me know.