Please help! Chronic left-sided headaches for three weeks...

Use your fear to look up info with a search engine. There are so many things it could be, and I don't think any of them are going to kill or disable you. Also, change that fear to communicate a sense of urgency to your doctor(s) that you want to get a confirmed diagnosis and treatment and prognosis.

You haven't said where the doctor left you. Did he say call him if it doesn't resolve in 'x' days? Did he tell you to try heat, or antihistamines or what? Get him to not just rattle off possible diagnoses, but to make you understand them -- take notes. Follow up with questions until you are sure you understand wht to do.

Do you have allergies? Taking any medicationss? For what conditions? How long has this been goig on? Do you have a runny nose? Is your mucus clear, cloudy, thin, or thick? What color if any? You seemed to be implying that you are able to sleep ok. Do you have post nasal drip?

What did your doctor say to do next?

Does it hurt if you gently press on the bone around the eye or above the eye?? If you try using warm soaks does it help? If so then you most likely do have a sinus problem. The biggest problem I had was the family dr who will only give you antibiotics for 10 days and that never helps if you have a bad infection. I ended up going to an ENT who put me on antibiotics for 4 months and it took that long to clear up the problem. So if you are not getting anywhere, go to an ENT.