Exes by definition are just that, past tense and should remain that way; you broke off the relationship to move on for various reasons, so do it.
This whole “were just friends”, and the “lets be friends” psychobabble is a product of too much daytime TV and people far too disconnected from the harsh veracity of the human emotion. Only so few can function within a relationship such as that where one or more individuals have interaction with past lovers.
My ex-wife watched Beverly Hills 90201 religiously and with each passing day I saw more and more her life begin to emulate certain mannerisms the characters all embraced on that show.
One was a conceited approach towards her past relationships and the fact that she expected me to openly embrace them and allow her to revisit them when she felt the need.
It may be nothing but if it bothers you enough to post here then you need to address this issue with your G/F before it dwells on too long.
Most of us know why we're alone...what we don't know is why we can't seem to do anything about it.