I also had measles as a child - my mother kept meticulous records - right after I delivered my son, the doctor told me I showed no immunity to measles and had to get an MMR shot, which he immediately delivered - I was shocked at how much it hurt... not fun after just going through labor...
There’s two kinds of measles. Maybe you only had one kind. If you’ve had it you’re immune for life. Just like mumps which I also had on both sides.