The OP’s academic record is extraneous to the discussion. Presumably, if the OP has contact with his ex-teacher and she indicates no interest, he has enough sense to get the message. We have plenty of teachers having extra-marital affairs with current students, so the notion that it’s impossible to have this sort of interest is ignoring reality.
And, the facts are that both are unmarried adults who are not in a student-teacher relationship.
There is nothing wrong with it despite the squeamishness that some feel the need to express.