My son who is younger than your firstborn is almost entirely sustained on breastmilk. So yes, it's possible.
Is it possible your DC is getting sick? When DS had what we assumed to be rotavirus, he wanted nothing to do with food. He was seeking his best food.
It also goes along with his teething; yesterday his 2 year molars (which have been working their way through since he was 18 months) started going again and he hardly wanted food at all again.

Is it possible your DC is getting sick? When DS had what we assumed to be rotavirus, he wanted nothing to do with food. He was seeking his best food.
It also goes along with his teething; yesterday his 2 year molars (which have been working their way through since he was 18 months) started going again and he hardly wanted food at all again.