Am I overreacting ?

The guy I’ve been dating has invited me out to ride bikes with him and this girl that he used to work with. I didn’t even know they talked. We’ve known each other for quite some time and I never heard mention of her. I took the invite but in secret I felt bothered, annoyed, territorial, and jealous. I instantly felt they were messing around behind my back and I know it’s not fair that I became so angry jumping to conclusions. I don’t want to feel this way. I don’t want to be that “jealous girl”. But is this normal? Has anybody else felt this way and how do you get rid of feeling jealous ?