So, I kind of have a relationship problem.
I'm dating a girl for about five months. We're not the type to be jealous, but when it comes to my ex-girlfriends she simply goes nuts. For example, I had lunch with a girl I dated back in 2013 (we started talking again a few weeks ago) and my girlfriend freaked out and got all angry. And now, I'm thinking about going to a show from a friend's band, and a girl who I had some "intimate relationship" for one night (if you know what I mean) is going too, as she is the same circle of friends. And guess what? My girlfriend is angry again.
Even tough we had that night, we are just friends from even before I met my girlfriend, and I don't want to stop talking to someone I trust because because of that.

Hmm. Do you love your Girlfriend? If you do, convince her that you'll only be hers, and convince her not to worry about other girls. because you consider yourself as hers, and no one is going to take you away from her.

On the other side though, if she is still not convince, it probably is because she don't trust you enough. She don't trust you as in, she don't know if you can defend yourself against other girls who are looking for a chance to be with you intimately.

It is difficult to convince someone who gets jealous so rapidly that your past relationships do not hold the same value as the one you are currently in.

First determine if her jealousy is in anyway related to some form of low self-esteem/recent incident that might make her feel insecure or if said jealousy happens because she has an entirely different perspective on the say you should be interacting with your exes (does she believe that because you are in a relationship with her, she is owed all of your attention?).

If it is the former, work extra hard on communicating your thoughts to reassure her of your attention and the importance of your relationship. Invite her to speak to your exes, if necessary/feasible, since it is easier to hate someone you don't know than someone you are somewhat well acquainted with.

If it is the latter, then you have little choice but to explain to her how you view things (put emphasis on the fact that, given your friends, it would be improbable that such meetings do not occur, even if you tried your best to avoid your exes) and see if there is any compromise to be reached.

If she is entirely unwilling to accept any of your arguments and there is no rational solution even after all the mutual efforts put in to deal with this, then I can only wish you the best of luck.