Feeling ill when I am hungry and pain a few hours after eating

Hi, I am been having problems for quite a while, when I feel hungry I also feel lightheaded and feint. 

Once I have eaten I am fine for a few hours or so but then I develop pains which go across my back which I can't get rid off and they can last for a couple of hours before I eventually vomit and the pain goes.sometimes I eat at about 7pm and the pain wakes me in the small hours So it is not like it is indigestion from eating too fast.

I was prescribed lansoprazole about a year ago who I have been told to take everyday and I can be fine for a few weeks and then can have a few weeks where it happens virtually every day.

Does anyone know what could be causing it ?