[Help] Severe pressure headaches, nausea and dizziness.

I've dealt with migraines and pressure headaches for a long time. When I started keto last January, the migraines disappeared and the pressure headaches were less frequent. In the past few weeks, I have been getting terrible pressure headaches.

On December 19th, I got a horrible headache. I had severe pressure in my head and my eyes, my neck was extremely sore (even tender to touch), I was dizzy, my ears were swooshing and ringing, I felt nauseous (and did vomit once), and felt as though I was going to pass out any time I stood up. It ended up lasting for almost 9 days. It was getting so bad that I considered going to the hospital (I had chronic daily migraines for almost 10 years, so I'm used to just pushing through things like this). I realized I hadn't been drinking that much water, probably 1.5-2.5L a day. I drank around 12L in 36 hours and woke up feeling absolutely fine.

Over the last few days, it has started to come back. The pressure is bad, I feel really dizzy and nauseous, I feel like I'm going to pass out even though I'm sitting down. I've probably had 3L of water a day the last few days, and I'm sitting here drinking more (probably around 5-6L today), but it doesn't seem to be helping. I just feel like I'm going to be sick and pass out.

Is there any reason I might suddenly be getting very easily dehydrated, or has anyone experienced something similar?

Also, I'm Canadian and in the US visiting friends right now, so if I can avoid it, really not wanting to go to the hospital.