First, I am SO happy to hear that the scan revealed nothing serious - although I can relate to your frustration (if not this - then WHAT). I've been having what sounds like something similar. I'm about 5 months pp and I've essentially had a headache for about 8 weeks now. At first my GP thought I was just tired, but now he thinks its a sinus infection (I really don't think it is - I have NO congestion at all), so I am on my second round of antibiotics, and on Monday, I got a shot of Toradol and I started taking valium (I'm not BF, so no worries there) to "break the cycle." If this doesn't work, we will start the imaging process. My headaches (well, headache really because it never goes away entirely) aren't debilitating, but just make it really hard to function and get through the day.
I have the same questions you have though: am I going through some kind of hormonal transition? I know the other pp things are in full swing - hair falling out, skin discoloration improving - so I just think this is related somehow. All I know for sure is this sucks (sorry to be negative - trying to be positive) because I can walk for exercise but I am a swimmer and I can't swim because I'm afraid I won't be able to get my head out of the water. I'm just believing it will get better when I get passed this transition.
Hope you find some relief, and if you do, could you please let me know? If I find anything out, I will post, too. Thanks so much for reading