Laurie- any chance you're low on iron? I know when I get my TOM, I have to take Fergon until it's over, or I am wiped out just sitting down, never mind exercising. Has it been very hot where you are? Could you be dehydrated? Are you sleeping well at night? Are you taking in enough calories? I had to increase my calories recently when my exercise picked up- if I didn't eat more I would have either binged or became exhausted. Eventually, once I am more fit, I will lower them.
It could be a number of things, or you could just need a day or two of rest to recharge your batteries. Sometimes our bodies just say "enough!" and need a break.