What Causes Severe Headache Which Continued After Pregnancy?

I have had headaches for many months now, they started over a year ago when I was pregnant, accompanied by nausea and all over aching (like a hangover). I thought it was pregnancy. After I gave birth, about 5 days postpartum, the headaches started with a vengeance and have been going on for 7 months now with a few days off here and there. I don't have a thyroid as it was removed in 2010 due to a goiter and we found a small amount of cancer in it upon removal. So, I thought it was my synthroid was too high. My thyroid levels are good now and still headaches. I have had many blood tests all coming back negative: lupus, liver function, kidney, pancreas, cortisol, blood counts, hormones, you name it. I had an MRI that showed an inflammation in the Aeration of the Petrous Apex, but doctors keep telling me this isn't the cause of the headache. I am seeing an ENT and just had a CT scan of that bone for him to see what's going on. He also gave me a mouth guard and told me I have TMJ. I have had that before and it doesn't feel like this. Other symptoms include sensitivity to cold, nausea, and joint aches (sometimes severe). I have also been tested for arthritis and it's not that either. Any ideas?