YAAAY another Paddy on the boards.
I gig 4 nights a week and then cover various sessions as well and it's very easy to slip into the trap of skipping meals or eating on the go from one gig to the next, the body can only take that for so long till you crash and burn. So make sure to sort out your diet and get adequate sleep (oh yeah and watch the gargle during gigs as well thats another killer).
(Another cause could be bad air conditioning and really hot stage lights, I've had my share of this over the years and it can get so bad you feel like your gonna pass out, a handy solution is to get yourself a portable fan and have it on stage with you).
best of luck.