I am 21 weeks pregnant and feeling sore in the tummy and abs area, is this normal?

That stretching and soreness you are experiencing is completely normal. While it is the muscles are stretching it is also the bowels being compressed, by the growing baby and you could be experiencing braxton hicks contractions. These are completely normal and even a critical component to delivery. The uterus may be "exercising" in preparation for the big day. It takes a lot of work to grow a new baby and the baby itself will litterally take everything from you, calcuim, all the minerals it needs even iron. Make sure you take a good prenatal vitamin and have the doc check your iron level at your next appointment. You could be deficient and that would leave you feeling tired. Good luck with the delivery and may you be blessed with all the joys of motherhood.