I think you ARE concerned — at least a little bit — or you wouldn’t have written in for advice. What you’re really asking is whether your concern is valid and if it seems to an unbiased person whether your boyfriend may leave you (or cheat on you) with this former fling. And I can’t answer that for you. What I can do is point you to the moment he told you that he’d hooked up with this former co-worker and ask what you think his motivation was for sharing that information with you and how he handled your reaction. Do you think he wanted to make you jealous? Did he simply want to be honest and open with you and give you the opportunity to back out of a relationship in case you were uncomfortable with the situation? Did he try to reassure you that he didn’t have feelings for this woman and that you had nothing to worry about? That early conversation can give you some clues about your boyfriend’s intentions and his relationship and feelings for this other woman.
That said, six months have passed and his feelings for you have likely changed — hopefully deepened — and it’s possible his feelings for the former fling may have changed too. Maybe her leaving the office reopened something in his heart for her. Or maybe it was a chance to really close the chapter forever on whatever they shared together (which could explain the sort of lingering hug you say you witnessed).
You may never know exactly what’s going on in your boyfriend’s mind and what his feelings are for this woman and if she may come between you eventually, but if you’re concerned — and you are — you should share your concerns with your boyfriend. HE’S the one to put you at ease here. He’s the one who can reassure you there’s nothing to worry about — that he’s not at all interested in having anything more than a friendship with this woman. And if he’s not able to do that, or if you’re still uncomfortable with the idea of a friendship between them — even one conducted in group settings — then after six months of dating, you may want to re-evalute whether this is a relationship you want to continue pursuing.
If you have a relationship/dating question I can help answer, you can send me your letters at firstname.lastname@example.org.