15months old got fever for 2days temp reach 101f, enlarge lymph node behind/mid left ear. no lab findings. pedia gave co-amoxiclab. what's the cause?

Probable Infection: The description sounds like an infection. Possibly in the middle ear or from a break in the skin. The lymph node responds to foreign material getting into the system to protect us from it spreading to the blood stream. When they are enlarged and the patient has fever it is a sign that it needs to be treated.

Fever in a toddler: Well it is important to identify the cause in a very young child like this. I hope your doc did a thorough exam -- did they look in the ears? Common things being 'common' maybe it's an ear infection - which would explain why they prescribed amoxicillin/clavulanate. Would try that and see how child does. Sometimes cause could be more serious so if any worsening -go back to see the doctor. God bless