Worn out!

I have been very tired and worn out lately. I have been chasing my youngest one non-stop. She just does not stop running around. I've tried taking her to a local library to attend the lap-sit reading program for her, but she has no interest in books. She is so different from my oldest. At 15 months old, my oldest one would sit on my lap and listen to the stories that I read for her. I think I lost a lot of weight as well for chasing my little one around. My little one usually goes to bed around 7:30p.m. and I have plenty of time to do stuff for myself, but I am so tired and do not want to do anything..... I need a break! Am I just getting old?