Question? Please help

my girlfriend (just got back together) says to me tonight 'oh btw im going to the movies with mark this week... right now heres the back story 'mark' is a good friend one i've never heard of till we've started seing each other again, also she tells me this at the start of a nice dinner at a nice restaurant.. I got a little upset told her how i felt about it and said she can make up her mind what she wants to do.. can i please have some input i dunno if i reacted badly i've never faced this kind of i guess jealousy, but also not trusting this'mark'

Easy and hard to work out. But if the shoes was on the other foot how would she react. This so call mark how can he be a friend if you have hardly know him etc. question you should asked yourself do you both have trust in one another. If so you both have to work extra hard to make this relationship work other wise you will always be looking over her shoulder etc