Aw sweetie
If he didn't have anything to hide and they were truly still "just friends" then he wouldn't have put her under the name of Jason in his phone. He is not being honest. I am so very sorry, but this is very fishy and I think he is up to something. I am also bothered by the fact that he let you leave like that only to just text you later. Then you go back inside and she is in there talking with him
. That is just wrong, IMO and I would be furious and hurt too
.