ex,ex's friends and i?

Here is the drama:dated this guy let's call him J and we broke up a couple of months ago(Feb. of this year).The guy J and I were best friends before we started dating so we knew each other for 2 yrs . Okay so here is the ordeal:one of my friend,Sabrina, is dating one of his friend Adrian.So they went on dates and kissed etc, not my deal but since I am friends with her when I hang out with her on Thursdays it will get messy if my ex is there. So Thursday of this past week her guy Adrian invited us to join him and his friends at this bar where my friend and I go anyways. We met up there,some of that guy's friends came and we chatted.Then a bit later more people came so we pull 2 tables together and sit down.So in conclusion I chatted with my ex's friends who are also friends of my friend Sabrina's date,Adrian. Anyways at one point this girl and I chatted,I found out her family owns this restaurant,we talked abt food,music,Mardi Gras and New Orleans.After her and her bf left, my friend and her guy went to the bar and I was stuck at the table with this guy.Anyways the guy and I talked and yes he is also a friend of my ex's.So in the end that guy made me laugh and smile and we chatted for a while and he was flirting with me.I am so freaking idk confused abt how things will turn out since my ex and I at the moment do not want to see each other or rather him.I am ok with being cordial with him but I am not sure why he acts like a jerk or something as though I am the devil.I am not sure what his friends knew abt me while we dated or after the break up but when they talked with me they got to see the real me:no games or strings attached.I am wondering if my friend and my ex's friend dating with somehow make this compliacted when I go out with my friend to the bar(my ex will probably think I will be stalking him or something).

So my question is: what can I do?I mean I am ok with his friend flirting with me and do not care anymore since my ex is acting as though he does not care at all.I mean should I stay away from where he hangs out?I put my life on hold per se after the break up for 3 weeks and wasted my time and tears and so forth.Now I am making up for that and I am painting,putting on weight since that is a goal of mine(am too skinny) and doing things such as country dancing which I do not do.Is that bad or does it show that I have moved on?