I Had a TT 3 Weeks Ago and All is Well, but I Wake Up Most Days with a Headache and Dehydrated

You sound like you are near chronic dehydration and I suggest you take in 8-10 glasses of water, and perhaps a little Gatorade for some electrolytes.  You may not have an adequate diet for recovery.

This could become a very serious issue. You need post operative blood/urine testing, and have your PS evaluate you for a metabolic issue, like anemia, diabetes, urinary issues. 

I recommend that you double your hydration.  Keep a big glass of water next to you all night to help prevent drying out.  You lose a lot of fluid through those drains believe it or not. If you experienced any vomiting after surgery then that can get you behind as well.   I'm not sure about your overall nutrition but sometimes patients who are low on protein have a harder time keeping fluid "intravascular" such that the fluid gets out of the circulatory system and settles in the tissues where it is not being filtered by the kidneys and recirculated.  Keep a low salt diet and eat protein, fruits and veggies.  If you are not feeling better soon then discuss it with your surgeon.

Three weeks after a tummy tuck is still considered an ealy phase of healing. You may be dehydrated from not drinking sufficiently postop as well as having lost fluid in the drains. You may have headaches from dehydration as wellas from anemia secondary to blood lossduring surgery and through the drains. I would suggest that you increase your fluid intake and return to your plastic surgeon for possible blood testing. You may end up needing to take iron pills.