Can high + ANA (1:640 speckled/dense) neg for all AI diseases (Lupus, RA, CTD, Sjögrens) be something other than cancer? 5 mnths PP with 3rd baby, weird neuro symptoms started 1 week PP, extensive W/U (blood & imaging), all normal except ANA & EBV.

Not cancer. High ANA suggests that you may be at a higher risk to develop autoimmune diseases but has nothing to do with cancer. If you have weird neuro symptoms, a neurology consult is likely indicated.
ANA positive. ANA are common, sometimes 5 out of 100 individuals tested. There are far fewer people who have autoimmune diseases. That means there are many, many people who have a strong positive ANA who will never go on to have an autoimmune disease. See a rheumatologist. Your primary care doctor may also recommend a neurologist, depending on your specific symptoms.