4 yr old with severe headache after taking antihistamine. no relief with paracetamol. crying nonstop. ?

I suggest you immediately take your child to an Urgent Care Center or Emergency Room. Your child may be having an allergic rxn to the antihistamine which should be evaluated & treated as quickly as possible. Your child may also be suffering from Covid-19, flu or some other serious condition which should be evaluated & treated as quickly as possible. I hope for the best for your child's health!