this is commonly refered to as "going flat" you get the feeling that your going to pass out.
this is generally associated with pulling to much fluid off or also if you eat a big meal anywhere up to a couple of hours before treatment.
if you are aware of it coming on yell and scream for a nurse, they should flip the chair back,give you 100/200 mls of saline and you'll be as good as new in a few minutes.