Why would the guy I’m dating tell me he bumped into his ex mum?

I asked him back in October if he was over the split and he said he was because we had a heart to heart conversation about it. However today he said he bumped into his ex mum and he said ‘ It was actually nice seeing her but I don’t think she would think the same’ part of me wants to reply back with why would she not think that? And another part of me Is annoyed his bought her up which is upset me because it’s not something I would want to hear oh it was good seeing my ex mum, seems his not over her and he shouldn’t really be dating me.

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by the way I meant HIS GIRLFRIENDS MUM! I have dyslexia so my spelling isn’t the best!
Why would the guy I’m dating tell me he bumped into his ex mum?
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