sleepy when working out

I recently started working out again. I lost 30 lbs about 3 months ago and although I've maintained it I wasn't doing my daily work outs, maybe just once or twice a week. However, about a month ago I started to experience extreme sleepiness while exercising. I went for a jog and right in the middle of it I had an overwhelming sensation to just lay down and take a nap. It was a bit chilly outside and my lungs didn't appreciate the cold air so I chalked it up to that. Since then every time I work out I just feel exhausted. My muscles aren't fatigued and I otherwise feel great, just extremely sleepy like I could just lay down on the spot and take a nap.

I haven't been jogging because it's still too cold out. When I exercise I use the daily burn program so it's a mix of of kick boxing workouts and cardio/strength training.

Does this happen to anyone else? Any ideas what could be the cause of this or how I can prevent this from happening?

I do generally feel tired most of the time despite the number of hours of sleep I get a night. However, nothing has changed with that to be the cause of this sudden onset of work out sleepiness.