Fall backwards while sitting. Flip too.

My son is 9 months old ,he used to sit steadily without falling since his 7 months . But suddenly he always falling backwards last three days and had fever too,fever is getting better after given few doses of paracetamol. He started partially crawling around a week ago. We put him few days into walker ,but managed to push with his back backwards . Now we did not put him into walker . I suspect in this direction too whether it could create any pain at his back. Coming to back to original question,is it very critical fall backwards while sitting suddenly? Is he need immediate doctor consultation or treatment? He can sit on our laps with our support.can flip too .sitting alone is concerned and crying while fall back ( surrounded with pillows ,no hurt to head ) Kindly advice .