Slight jealousy over my ex’s new girlfriend?

I am in a healthy 3 year relationship, prior to that, i had a thing with a guy (first love) for 4 years on and off until i went to study abroad.
We never officially dated tbh, it was quite complicated and confusing. We never even had a ‘break up’, i just moved abroad and that was it, we are on good terms and we have moved on with our lives.
I have fallen out of love with him 3 years ago, i never think of him or write of him. He’s also in a relationship and everytime he posts i get this slight grip on my heart which passes away in a second and then i forget all about it.

Why could this thing happen? Maybe because we didn’t have a closure? Has it ever happened to you?
Slight jealousy over my ex’s new girlfriend?
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