날 갖고 노는 거야? = (너는) 나를 가지고 노는 것이야? (in full form before contraction) => Are you treating me like a plaything? / Do you mean to treat me like a plaything? * 갖고, 가지고: from 가지다, which means possess or keep. Literally "holding in one's hands" or "keeping on one's person". * 놀다: play; have fun. * 가지고 놀다: play with something (like a child with a toy). When extended to adults, it means 1) to treat someone selfishly like a plaything, showing no due respect, or 2) (in sports and competition) overwhelm the opponent so completely as to look like they're playing with a toy. * -ㄴ/는 것이다: It is that ... - an indirect form of statement commonly used.

R u playing me? 날 = 나를 = me (object) 갖고 놀다 = 가지고 놀다 (play. eg. You play with toys, dolls) ~야? (Question) It sounds very negative, implying blame.