Is my ex boyfriend (who dumped me) going out of his way to keep tabs on me? if so, why?

I haven’t spoken to my ex in MONTHS. Didn’t even have him on social media when I decided to block him (just so I wouldn’t be tempted to look at his posts/to help me move on).

A couple of days before I blocked him, he added my sister on Facebook, 3 months after she sent the friend request (has never met my sister, he won’t even have HIS own family on Facebook).

Literally hours after I blocked him, his flatmate (whom I’ve never even met) is adding me on Facebook. I’m not too sure how close my ex and the flatmate are.

Yesterday I told my sister about the situation and she says it seems like he asked his friend to add him to keep an eye on me. She also mentioned that my ex followed her on IG a couple of days after I blocked him.

Is it just a coincidence that his flatmate adds me the same day I block my ex, and my ex is following/adding my sister on everything, or does it seems as if something is going on?

I just don’t get the sudden change, from ignoring my platonic attempts to reach out, to seemingly doing everything he can to keep an eye on me :/

I’m trying not to overthink it, but it seems strange.

In regards to why this is all strange, he was the one who ended the relationship, didn’t think we could work on it, and ignored any attempts I made to be platonic (was the one to delete me on social media in the first place when he was drunk/angry).

We were friends for a bit after the split, but ended up hooking up and decided to “give each other space”, hence what lead to the not speaking.
Is my ex boyfriend (who dumped me) going out of his way to keep tabs on me? if so, why?
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