What Causes Crying Face In Babies?

My son is 7 months old and when he cries his bottom lip pulls down much farther on his left side. At first the neonatologist said to watch this closely but his current pediatrician called it his crying face and it is common. If this is the case, do I not find any search for a baby s crying face and should I watch for other neurological symptoms. He was born at 32 weeks gestation due to superimposed preeclampsia and premature rupture of membrane. He had a small brain bleed seen a week after birth but at a 4 week later ultrasound it had healed.