Hi Unique2Who,
Your case sounds very similar to mine - I am 24 and I have had a constant headache for nearly four months now, it started out of the blue and I have even had to take some time off work to try and get it sorted. I have been off work for one month now and understandably am starting to get frustrated with the whole situation. I have no symptoms other than a constant 24/7 pain in my right temple - the pain varies in intensity, sometimes causing me to cry because it is so strong. Nothing allieviates the pain and I, like you, have done loads of blood tests, CT scan of my sinuses (have an MRI tomorrow but not feeling promising), saw an ENT, had my eyes checked, etc etc and everyone keeps on saying I am fine which is very frustrating because I am not. I even saw a Chinese doctor out of desperation with Western medicine and did acupuncture too. Nonetheless I still have the headaches. I also saw a chiropractor. Lately I have started seeing a kinesiologist - and we seem to be getting to the root of the problem. The thing is you need to find someone good - I am happy to recommend him although I live in the UK and I am not sure where you are based?
Also, today I received some blood test results of other blood tests I did last week (was having a look for food allergies which I recommend you do too - the kinesiologist suggested to stop eating nightshade foods, have a look on google) and they said my kidney doesn't seem to be fully functioning. Bizarre as this had not come up in the numerous tests I did in the past few months.
Everything in your body is conncted, it could be anything ... which, trust me, I know is even more frustrating! do you have any other symptoms?