My 3.5 year old is congested so i gave her some benadryl to help dry her up but she has started coughing which is preventing her to sleep really well.

Needs 2 b seen: Kindly take her to the pediatrician or to her doctor/urgent care, she might have reactive airway disease in addition to upper respiratory tract infection, she may very well need Rx medicines, all the best for you both

It happens: One effect of the antihistamines like Benadryl is thickened secretions. It is easier for them to cough them up if they are thin & loose.We find that these preparations seldom help kids with congestion anyway & don't recommend them under 6. Her illness may also have intensified to the point she needs prescription meds.