Thank you both @afrobin and @catmom777 (cool nick by the way, cats!).
I don't think it's low sodium, I don't really have a low sodium diet. Something changed since about two weeks ago, and I have been feeling increasingly sick. But the brain swelling is interesting, I am used to drinking quite a lot of water, ever since I was young I would start my day with half a liter of water, then half a liter of tea (French roots, we drink 'un bol' [big breakfast bowl] or more for breakfast) and drink more during the day. But I don't think there is a relationship now. I did notice however that lately, even after drinking some tea, my head would feel like it was exploding and the pulsating sensations grew stronger. I will stop taking the magnesium for now to see if this will help me feeling better.