im just trying to help. most people rely to much on caffeine and you start being addicted, in small words, "we"(your body) build resistance to caffeine, in order to protect himself, against external substance that prevent "us" from relax/sleep. after this test limit your caffeine intake ( dont cut it all out at once) just reduce it, and some days go off , when you get back with some coffee you will see the beneftis again. I already went through that phase and trust me it's not pretty. I felt the same way , caffeine "stop working" in a benefetial way, and i was overdosing myself, and feeling even worse, until i reach around 900mg and had tachycardia attack ( pre- heart attack, you start sweating and you get really cold inside, you heart rate/beats per minute raise really fast and high).
Just study get your self a drink with 200mg-300mg caffeine , and next days avoid cafeine after 5-6pm if u sleep around 23h. and never go above 500-600mg