Why is he mad at me right now?!?

It is obvious Tom is trying to creat trouble for your relationship with Nate! "Tom always shamelessly flirts with me and tells the most humours scenarios of us getting together, but truly I know this is all in fun and jokingly." You should avoided this although you know this is all in fun and jokingly. If Nate is always shamelessly flirting with another girl, what will think?