I think it is normal! Your body is working so hard to make this baby, you have hormones out the wazoo, and you are about to undergo the biggest change of your life (becoming a parent)! Big stuff!! So having lots of emotions and even some that worry you because they aren't totally positive (like I worried if I was making a mistake even though we tried for 3 years to have our first baby . . . like really? LOL I wanted a baby more than anything and even *I* had that thought run through my mind!). It shows you are thinking about things realistically and in a practical way to have occasional worries. being a parent is hard work!! But I'll tell you, the most amazing and wonderful thing I've ever done!!
So, try to do things you find relaxing and chalk it up to hormones and the normal thoughts that run through our head when pregnant. good luck and hugs