Low placenta

Today i am 21 weeks had my appointment with my ob doctor and she said from my ultrasound last week it says i have a low placenta. About 1.5 inches low than usual. Im hoping in about 5 weeks for my next appointment it will be up more. It just makes mw nervous. She said it could be stress related and i cannot think of why i would be stressed. But she also said baby is healthy and heart rate at 136 and my baby bump is measuring out normal. :) i love my baby girl. So i i dont think i have much to worry about. Is low placenta pretty normal around 21 weeks for some women?:)