If I try to write now it's barely legible. Could it be the brain damage from stroke sneaking up on me?

Signs of stroke. Could be. If this smptom came sudenly you need to go to the emergency room. If this is gradual, get evaluated by a neurologist. Remember that strokes can be avoided or reversed if treated in the first 4 hours of symptoms. Micrographia ( write with small letters is a sign of parkinson's disease.
Yes. This could be from cerebral damage, dementia, parkinism or the normal hand writing of a physican.