9 month old now refusing to sit up

My baby is almost 9 months old and he has been able to sit up unassisted really well- he would enjoy sitting up and playing with his toys or his friends at daycare. We would usually put a boppy behind him just in case he lost his balance but mostly he did just fine. However, about a week ago with no reason I can think of, he started refusing to sit up. I place him down and he immediately arches his back and flops backwards, crying. He will sit in his little skip hop activity center , but he just refuses to sit up on the floor or even if I try to sit him up on the couch, he will do the same thing. Has anyone else experienced this? It is just a phase ?