How hot is it where you are? We big gals have a hard time dumping heat, so if the day is on the warm side, that could cause general ickiness.

Also, if you are riding for 30 minutes, don't bother eating especially for the ride. You can live off the fat of the land for half an hour. I've found that riding on a full stomach is just yuck. I try not to do it. Don't forget to drink, though.