It is possible that 2 different infections are present in the body at the same time. This is especially likely in cases when a viral infection (glandular fever in your case) decreases immunity and makes the body susceptible to develop other, usually bacterial infections. For example, people diagnosed with glandular fever are susceptible to developing bacterial throat infections.
Glandular fever can be identified using a specific blood test (which was positive in your case). A sinus infection (sinusitis) can be confirmed or excluded via nasal examination (rhinoscopy) by an ENT-specialist. You may also request an X-ray image and microbiological examination of the sinuses for further diagnostic information. In your case, none of these examinations have been performed and you're assuming that a headache in the sinus area indicates sinusitis. Headaches, however, are very non-specific symptoms because a headache can be present during many diseases and conditions.
For a precise medical and scientific diagnosis, you can consult an ENT-specialist about these symptoms.