ex boyfriend, now best friend

I have this amazing boy in my life who i dated when i was in 7th grade (it was a middle school relationship which didnt really mean anything at the time) and after we broke up we stopped talking because it was awkward. For the past two years we've been getting closer and now im happy to call him my best friend. These past few months we've been through a lot together and just as he's been talking to this girl I've been getting very jealous and I don't know if i just am getting jealous that she's replacing me as his best friend or that I have feelings for him. Some advice would be greatly appreciated!