I had measles as a child (rubella & reg) yet turned up w/ no immunity on work bloodwork. How could that be? Gave me MMR vaccine am I know immune?

Mimickry. You did not have a true measles, instead, had some febrile illness with rash. It is not uncommon for the lay people to misinterprete other viral rash as measles.
You are of. An age that you "missed" the measles epidemic of the mid 50s. The definition of "known immune" would be evidence of immunity on blood work, so reimmunization is appropriate.
Yes. Immunity can wane over the yrs. Assuming yr immune system is intact and u are healthy, then an mmr-ii. Would now make you immune.