Just before that Ross heard an answerphone message where Rachel said she was 'over him' i.e she no longer had romantic feelings for him. Being 'under someone' isn't a figurative expression in English. This phrase is a joke coming from the previous section of dialogue. If now Rachel is 'over' him, then at some point she must have been 'under' him ('under' being an antonym of 'over'), that is to say she had romantic feelings for him at some point. So Ross is asking when did she feel that way.

its not something that makes sense actually. its kind of the joke there. rachel said that she was over him. which means that she wasnt in love with him anymore. when ross asked when rachelz was under him he meant to ask when was rachel even in love with him. the joke is about over and under. its not the commen sentence we use in english really

That's very poor slang and doesn't seem like the type of thing that would be said on Friends. Although admittedly i dont really watch the show. Correct grammar would be, "when were you under me?" The would most commonly refer to either a sexual position, or a business relationship (to be under someone means that the other person has a higher position in the company).

I realized one more mistake . Should I write " had feelings for him" instead of " had a feeling for him " ?