Ive had these before, (atm im not VLCD just low carb) but when im in full ketosis and VLCD I get them a lot, nearly all the time.
I think it might have been the stomach shouting for help or something.. it was worse when I was due to have a pack.
drinking more water helps and using a hot water bottle and having a lie down.
I was never fully sure what it was but I didnt have any health problems at all, it didnt get worse and went away when I started eating again. -then came back when I stopped, then went again..!