I dont know why but i feel really jealous?

today at school we had school trips for what ever section of the CCF (combined cadet force) u r in. im in RAF, and my bestfriend is in Navy, and me and my friend normally spend loads together.i had a really good time though but when i got back my friend said she said it was the best time she'd ever had and she spent it with her and mines ex best friend and this other girl who she wanted to not be friends with anymore -but has now changed her mind, when she said that it got me kinda upset as i felt it was the time she had with out me that she enjoyed the most..idk why i feel like this , i just do :/ is it normal? or am i just being mean?