Workout problem

I finally got up the nerve to join a gym.  So I am not equipment savvy yet and kept with the treadmill and then added the bike.  For the first couple of weeks I had no trouble with the tread.  Most days I did at least an hour a day - coming out dripping!  Not exactly a speed demon - 2.8 - 3 settings but felt like it was a good workout.  Then this week I got on and 20 minutes in got very light headed and had to stop quickly - felt like I was really going down for a moment.  Went home and checked blood sugar which was 115 so not bad for late afternoon.  But the next day I tried having a South Beach protein snack bar - 100 cals., 7 gr. protein, very low sugar (thinking less than 2 gr.) just before the gym. Got on the tread and within 3 minutes had the same things happen.  Got off and went to the bike which was fine as I was seated.  Had no problem with that and did just 30 minutes.  So what is happening?  Getting lots of water in so don't think its dehydration and the second day hadn't even worked up a sweat.  Walking itself didn't have that effect either.  Is it the motion or something else.  I love the tread and don't want to give it up but really scared that I might not be able to stop in time if I get light headed again. Its a 24 hour gym but at times no one else is there.  Any ideas?

Exercise induced hypo-glycemia? Google it and see if the symptoms fit... if they do, there are some tricks for managing it.

I will also so that in the first 6 months a lot of us experience all sorts of weirdness and we mostly outgrow it so it's probably a temporary problem. If it doesn't go away on its own though, you do need to talk to your doctor. But I bet it does.