Honestly? Yes, I think so!
But I don't think it has anything to do with the diet...
Most headaches are stress related and 'tension' headaches as we clench our muscles in our skull when we get worked up... I would say it's probably nothing more than you've worked yourself up into having the tetra (hearing how others rave over it, what if you don't like it, what's it gonna taste like, am I gonna find it vile, etc etc etc) and then it's a post-event reaction... am sure it will pass and the next one won't be such a big thing... Plus our bodies are suddenly being given all this water that it's not used to so it takes a little while for it to process and catch up to the idea of this being a good thing...
How did you like the taste? I find the choc tetra yummy, I could live off those for a week..! But one thing I find amazing on this diet is how people react so differently to the flavours - I for one hate the soups with a passion even tho in 'real life' food eating I am a savoury eater, yet I find all the sweet shakes really nice, some hot, some cold... taste buds eh, funny little things!
Keep going, am sure you'll feel a-ok really soon!
tinks x