Hello and hope you are doing well.
It is probable that the fluid in the ear is infected, this could cause a middle ear infection. This can spread causing a painful lump behind the ear. This could be a lymph node or sometimes the infection can spread to the mastoid bone. The condition needs proper evaluation. You will need to consult your primary care physician for further evaluation, who may then refer you to an ENT specialist.
Hope this helped and do keep us posted.