Won't sit anymore ? My son is 9 months next week, and since he was 6 months he's been able to sit…

Won't sit anymore ?

My son is 9 months next week, and since he was 6 months he's been able to sit unaided. Over the last 2 weeks though he just won't sit. If I try to sit him on the floor he'll push his legs out and won't bend. If I do manage to get him into a sitting position he just pushes back against my hand so I can't let go. He will sit on my lap though, and he will balance on my lap. The only reason I can think that he won't sit up is that he did have a couple of tumbles and maybe he hurt himself and it's put him off. Has anyone else's LO gone through a phase like this? Or has anyone got any suggestions as to how I can get him to start sitting again?