Carrying low

Yesi • Artist Happily married, with one beautiful son
Hey guys hopping to get some opinions here. 
I had my second 20 week ultrasound on Tuesday and yesterday I had a call from my doctor who tells me I'm carrying low. She explained a lot of it but it didn't really help me understand much. She said I'm not too low but that they want to keep on monitoring the baby so I have another ultrasound in four weeks. Have any of you been told this? Anyone had any experience with this? I don't worry easily but I'm starting to feel a bit uneasy.