See, I don't think that you've written her as being OOC because a) neither the manga nor the anime shows us what would happen if the shitennou came back and how the individual characters would deal with it, meaning you have a lot of room for interpretation here and b) Makoto always wanted a boyfriend, she never wanted to end up alone, so now she has found someone, and then in cames Nephrite, whose mere presence alone threatens to turn her whole world topsy-turvy and then they even kiss, so she's about to lose the man she's with for a man she cannot be trust because of their past. So for me, her anger works, it's understandable. It's highly unfair of course, but understandable.