What Causes Headaches While On Sudafed For Ear Infection?

Hi, I have recently had ear infections that have been prescribed antibiotics for. After completion of them I returned to the doctor to b told I have fluid in both ears and to take sudafed. I take the sudafed and it fixes it but when I stop the pain starts again. Last night I woke with a terrible headache in my right eye which has persisted all day even with panadol and my right ear is excruciating