Intolerance to milk sugars is a common cause of chronic diarrhoea
.But since the stool examination is normal this is unlikely in your child.
Persistent loose motion
is a common presentation of HIV
infection in adults but not in infants.In infants failure to thrive
,hepatospleenomegaly are more common symptoms.
The most likely cause for this diarrhoea is following two:(i)As a side effect of cotrimoxazole.In this case the benefits vs risk ratio needs to be calculated and a decision reached accordingly regarding continuation of the drug.
(ii)As normal variation in infants in whom the GI tract is not well developed.No treatment is required here.
However,since the child is feeding well and gaining in weight,no serious illness is expected and hence you can be fully free of tension.