I try to force myself to take a walk...whether long or short...after work if the weather is nice. If the weather is bad...I walk up and down our stairs to our basment about a dozen times. Exercise always energizes me...it's the getting energized to do the exercise that's the hard part. Ha! But, every little bit helps I guess. I know I always feel better the more exercise I get. Even if I'm just doing dishes or laundry....anything to keep moving. Like my 75 year old mother-in-law says when she keeps moving..."use it or loose it!"
You might also have low blood sugar...if you can, try to eat some fruit or juice or something like that. That helps me, too.
Do keep an eye on it though and see your doc if the exercise doesn't do it.