Why does my friends with benefits acts jealous?

He was the one who wanted to initiated friends with benefits situation. At the beginning he was not jealous, he told me that i could meet other guys if i wanted to. But in the last few months, he has changed. When i talk about other people that i know (work collegues or friends etc, so not about dating someone' else), he is very jealous. Why has he changed? Furthemore, he did not call me first for a very longtime. Lately, we did not see each other at all. Now, after i said that i really care about him, he has started to call me first and acts more relaxed. But he is still acting jealous. Why? It’s such a mess, if you can help me understand I would really appreciate

Also, sometimes I publish on instagram pictures that i dont publish on fb. He doesn't have instagram, but i have noticed that his best friend is following me now. He can see if i publish some new pictures. Why?
Why does my friends with benefits acts jealous?
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