Why does my bf randomly choke me?

Background: I like it during sex but not too hard to the point that I am really struggling or going unconscious.

Now: now that we live together he just does it randomly and continues even after I say stop. Like just out of no where. Idk if this is just playful but I wish he would stop when I say stop. I can breathe fine but it’s more of like being constricted. It causes a lot of strain on my neck. I’m already on my computer all day for school.

When I tell him stop he just says “you can breathe” or “I’m not hurting you.” And gets mad when I push him off of me. Like I tried to tell you to stop and now the next step is to push or fight you off duh.

Anyways, should I be worried? Why does he do this? Help.