Dizziness , blurred vision , pressure headache

I've been experiencing dizziness, blurred vision, & a severe constant pressure headache since Tuesday.
I saw a doctor on Thursday who told me he believes I have Sinusitis(for the second time this year)although many of the symptoms I had last time aren't present this time. I haven't recently had a cold. I don't have a blocked nose. I have no mucus at all & I have no loss of appetite.

He prescribed me Doxycyline 100mg(which is what was prescribed for me last time)& told me to take 2 capsules on the first day & then 1 capsule everyday until the end of the course.

Since then I have developed swollen eyes, with very red, sore, puffy and peeling/dry skin underneath them. I have also become very lethargic and weak feeling. My original symptoms are still present. I have also found that when I eat(especially sweet foods) my headaches and lightheadedness seem to ease or virtually disappear for an hour or so, before returning. Is this linked to Sinusitis, medication or something else?