insulin spikes wouldn't do that
adrenaline doesn't kick in until a short while into exercise. since my physio notes are out, i double checked and it does take a little while for cardiac output to increase to meet the needs of exercise so feeling lightheaded may just be part of the adjustment
are these just as you get off the bike?
take the gel 10-30min before the run, not 1hr. an hour will leave enough time for insulin to spike, leading to uptake of glucose. typically this overshoots by a little and results in depleted blood glucose as most of the glucose is transported into the cells. (and if you have been sitting around some is used to make fat. the insulin spikes are also not good in the long run -think type II diabetes). it may be that you feel better after a little bit because the glucagon has kicked in and you are releasing glucose from your liver glycogen stores. in practice, unless you are working out in the morning (and skipped breakfast) you really should have sufficient glycogen to last 1-1.5hrs and a gel isn't necessary at the outset of a workout. another case where it might be helpful is in the case of a second workout for the day (when the previous workout exceeded 1hr).
i know there is at least one physician on this site?
if you are worried and not really young (college age) it might be worth getting checked out.