Bf friends with his ex

They only dated for less than a month but still keep in contact and they have hung out a few times . We been together for 2 years and I've became friends with chick , no lie it's weird knowing they had a thing ( hooking up whatnot) so I try to look past it since their still good friends and it's of the past. Any one ever became cool with your bf/gf ex and how do u feel about situations like this ?