Do you have a heart condition? If you do, stop waterfasting immediately.
Anyways, during long waterfasts you should rest as much as possible. Some people can work no problem, apparently you are not that lucky. You should probably take some time off work or just break your wf.
I'm on day 26 of my long wf, I've had the same symptoms in the first 2.5 weeks of my fast. I would just lay down and wait until it'd dissappear.