Malarial parasite is a bitch. It can live inside liver cells, without having to take over RBCs. I don't know about how hospitals are in Nigeria but try to get the tests done and stay at hospital till they figure out what is wrong, if possible. Maybe it is just vomiting because of some contaminant in food. It maybe because of medicines too. But it is always best to stick to the side of caution, IMO. Wishing your young one a early recovery.