secondary to infection is a common occurence.
I hope the bug that caused the food poisoing illness has now been identified and treated.
Urticaria can be treated succesfully with long acting antihistamines like Allegra
but needs to be taken for 6 weeks after an infection.
Non-sedating antihistamines (like Allegra) is better than sedating ones as he is already exhausted.
Please ensure that the entire symptoms of rash, abdominal pain
are NOT secondary to viral infections such as dengue virus (as it is now quite common in several parts of India).
Dengue IgM test will let you know if currently infected along with NS1 Ag but needs to be done withing 5 days of illness. A CBC with platelet count
is also mandatory.