Babies Hair Falling Out

My little guy was born with a full head of hair, as time has gone on he's lost some of it (understandable)...But in the last 4 days he is pretty much balding on the left and back of his head.  Losing lots of hair on the right side too but not bald.  Monday he had his 4 month visit and by Thursday he woke up with way less hair.  I'm sure the hair loss has been gradual but I'm not kidding his head is like smooth and soft bald.  I'm finding hair on his sheets and clothes.  I called his pediatrician and she said it sounds like it maybe baby hair but that we should keep an eye on it and keep her posted.  Is this common?