+opk 11 days ago; postpartum 7mths, still in amenorrhea, no longer breastfeeding. Is it a false +ve?

Probably not false +. Most ovulation prediction kits (opk's) fairly accurately detect leutinizing hormone (lh) surges, which occur just before ovulation. While it is possible to have a positive OPK from an LH surge and then not ovulate, it's more likely that the detected LH surge was followed by real ovulation. The first day of the menstrual period should be 14-16 days after a positive opk.
Likely correct pos. It is very possible to get pregnant before you have a period again.