Boyfriend was planning on taking another girl to the cinema? :(?

im 18 hes 19 and we've been together for 5 months and we are great together and I know he loves me and i felt like i could really trust until recently he had been on my facebook and read all my mail (there was nothing on it cos I dont cheat) and since he done that i read his mail and noticed that from the day i was going on holiday, he had been talking to another girl (a munt) who id never known about before and I had noticed he asked if she wanted to go to the cinema with him and she said yes. I wouldn't have been so bothered if they hadnt been calling each other gorgeous etc :S when i comfronted it with him he said it was a big mistake and he was soo sorry and hes deleted her num and everything but i feel like i cant trust him anymore but he didnt seem the type to do this :/ im not the type to get jealous and i know he has girl pals and i could trust him to go out with them for nights out etc but now i dont know what to do :( am i just being para? help!