Aw mamsh, normal lang yan actually, maraming babies ang nag-shake ng head parang "no no no" lalo na pag excited or playing, ganyan din ang ginagawa ng baby ko around that age! But since 9 months na siya at you're noticing it more often, mas safe na i-check mo sa pedia mo lang para peace of mind mo, kasi alam mo naman ang mga doctors mas nakakaintindi ng kung normal developmental milestone lang o may kailangan i-monitor, one of my mum friends din worried sa similar thing kaya dinala lang sa check-up at everything was good, pero mas okay ang feeling once na na-confirm ng doc. Keep observing din kung may other behaviors that seem off, but I'm sure baby mo is just being a normal playful 9-month-old!