That is baby colic, not uncommon among premature infants.

https://en.wikipedia.org/wiki/Baby_colic

Baby colic, also known as infantile colic, is defined as episodes of crying for more than three hours a day, for more than three days a week, for three weeks in an otherwise healthy child.^[1] Often crying occurs in the evening.^[1] It typically does not result in long-term problems.^[4] The crying can result in frustration of the parents, depression following delivery, excess visits to the doctor, and child abuse.^[1]