The past few rides I've participated in, I have really struggled physically during the first 5-10 miles. What I mean by this is I will sweat profusely, feel a bit weak/lightheaded and feel just overall BLECCCH. After I get past this, and usually after my first rest stop for a Clif bar, I am O(plenty of energy and not sweating near as much) for the rest of the ride for the most part as long as I stay hydrated and eat. FYI, I have been eating oatmeal/toast/OJ for breakfast before I leave out. Time between that meal and when I first start the ride has been ~an hour both time.

Any suggestions what this could be?