Kathy,
I think the tightness that you feel may be your neck muscles. Your head is in a cocked and weird position during surgery and if you had a large AN (as you said) it's likely that your head was in that position for a LONG time. My surgery lasted about 12 hours and I know exactly what weak neck muscles feel like. Fortunately, with time it gets better and you can help things by putting on a warm compress, or getting a neck massage (if your doctor allows it).
I think Jan is right about the lumbar drain.. In addition, if there is a built up pressure due to CSF build-up (one of the effects of that is headaches, btw) then they can utilize the drain to relieve the pressure. Think of CSF as fluid in a constantly moving stream of water inside of a circular hose.. Your body constantly absorbs and recreates CSF to keep the flow moving.. Well, what if you produce faster than it's absorbed? The "hose" will build up pressure points and, in part, give you headaches.. As soon as your body settles in and stats absorbing / creating CSF on a more even keel, then you won't get those build ups. At least, that's the way I understand it. I am not a doctor and I don't play one on tv.
Being only 3 weeks post op for a "large AN" then I expect there to be some amount of "settling in" that your body is still having to do... I also had a build-up of CSF (which caused some headaches) and my doctor put me on Diamox (a diruetic) to bring the pressure down. Little did anyone know that I would have an allergic reaction to Diamox that landed me back in the hospital for 4 days at about 2 months post op. So, I wouldn't suggest going the Diamox route just yet.. Try to control the headaches with OTC drugs and if there is any kind of swelling of CSF around the incision area, then you can try an ace bandage wrap around your head (like a headband of sorts) to keep pressure on the area, control the CSF production a little bit, and wait it out until the CSF production becomes on more of an even keel.
Definitely mention it to your doctor at your 1 month follow-up. See what he thinks based on what you say, what he sees and what he knows about your particular case..
Good luck.. Recovery has it's ups and downs, as you're finding out.. Keep us updated on what the doctor says..
Regards,
Brian