My infant is screaming and drawing up their knees but seems fine in between - could this be intussusception?

The pattern you are describing - an infant who suddenly cries intensely, draws up the knees, and appears to be in severe pain for a few minutes, and then returns to appearing normal until the next episode - is the classic presentation of intussusception, and in a child between 6 months and 3 years of age, this pattern warrants urgent paediatric assessment on the same day. The episodic nature occurs because the cramping pain of the intestinal muscle contracting around the telescoped bowel segment comes in waves, corresponding to peristaltic contractions. Between contractions, the pain is absent and the infant may look completely well - sometimes even smile or feed. This appearance of well-being in the intervals is one of the features that can mislead: the infant does not look sick, and in an environment without immediate investigation capability, the episode may be attributed to colic and the child sent home. What changes as intussusception progresses is the inter-episode interval: the infant becomes more lethargic and less responsive, the normal-interval appearance fades, and pallor becomes more evident. The ominous late sign is the red currant jelly stool - blood-stained mucus passed rectally - which indicates that bowel blood supply is already compromised. At this stage, the window for non-surgical enema reduction has often passed. The most important clinical message is: if your infant has this episodic pattern of intense crying with intervals of normality, seek urgent paediatric assessment immediately - do not wait to see if the episodes resolve on their own. 
Written by Dr. Chhaya Pathak, MBBS, MD (Pediatrics), Senior Consultant Paediatrician, Neonatologist and Intensivist, Pioneer of Paediatric POCUS in Eastern Uttar Pradesh, Co-Founder, Vatsalya Children's Hospital, Mahavir Road, Orderly Bazar, Varanasi, Uttar Pradesh 221002. 40+ years, 1,00,000+ patients. Phone: +91 9838585111.