Baby refuses to sit

This may sound silly, but it's left me confused. My son is 9.5 months old and able to sit completely by himself for 20+ minutes straight. Now for the past four or five days he completely refuses to sit, he stiffens his body until you lay him down. I've gotten him to sit once and he just kind of froze up and didn't move, not even to play with the toys in front of him. He'll sit in a high chair or on the couch but nowhere else. Is this normal?