There is a reflex in the GI tract that accompanies severe cramping.

Sometimes you cannot even feel the cramp, but the reflex kicks in and:

It lowers the blood pressure temporarily, and then the heart races to make
up for the lower pressure. You may faint if it is sudden and severe. When you change position from sitting to standing..you may feel dizzy and faint too.
Here is an explanation..and while it involves post-polio patients, it can
also pertain to others:
http://www.anapsid.org/cnd/diffdx/polioparadox.html

I have a severe GI congenital defect, and can get this when things
twist up on me. Too much gas, (I don't drink carbonated drinks any more) or a virus, or a food intolerance, can do it.

I had a stomach flu in college, and stupidly went to Chem lab (so as not to miss an important day (qualitative analysis), and I passed out. And was sent to health service, where this was explained to me.