Why did my ex just ask me if I had slept with anyone else if we broke up over a year ago?

My ex and I broke up because he decided to move away. Anyways we have recently been in contact just as friends, (we have seen each other once) however, I have moved to a very nice apartment and wanted to show off essentially... nothing more. He then told me he didn't want to mislead me and I was like "yes, I know we are just friends. Forever." He then asked me if I had sex with other people, and I did not tell him because? Why would I do that...

Anyway two hours after the conversation ended he texted me again about my instagram and going on it to "see if I had sex with other people" I then mentioned how would you know that from Instagram. Then he said it was a joke...

Like what? Is going on here? Never happened to me before.

Why did my ex just ask me if I had slept with anyone else if we broke up over a year ago?
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