Almost certainly due to caffeine withdrawal,
Suggest phasing out caffeine slowly while keeping the rest of your new diet! Well done,
I wouldn't do more than a 1 cup/day reduction per week, this is what I've been advised by medical professional recently.
Important thing is to measure your intake so you can reduce it, rather than not know how much you've drank.
3-4 cups/day is generally fine, and certainly isn't linked with a reduction in lifespan. You may get anxiety/stress symptoms from too much caffeine. (>4 cups, but will vary by person)
If you like the hot drink, go with decaff tea for one or two cups out of your cups a day (later in the evening makes most sense) - I find it tastes pretty much the same.