What Does This Mean?

What does it mean if hes kind of ghosting you, being nicer, and stalking other people.

Soo I have this fwb for the past year. (Hes ny baby daddy, been together for 3 years, and remained FWBs the past year after the relationship ended) While hes the only one I been with hes been with 5 other girls. So a year later I finally slept with someone else. Were open about everything so I told him the next day and he said that he was disappointed. Hes not mad because he has no room to be cause he has slept with other people, but disappointed it was with the person that it was with. Ever since hes been ghosting me at night (not wanting to talk to me and reply back to msgs) and he ended up looking up the person I slept with on FB and stalked his profile. And now hes stalking my family. Last night he looked up my sister on FB. And in general besides ignoring my msgs hes been acting a lot nicer than he normally does. What does this mean? Am I "inside his head" now. Idk whats going on inside his head. Is he extremely jealous now? Like yeah. Advice please.