My 3 mo old wakes up screaming like something is hurting or scaring him...what could cause this? after a minute or so he usually goes back to sleep

My money's on reflux: A pain cry in a three month old may be related to gastroesophogeal reflux. Food from the stomach sloshes up to the esophagus, which is less protected from an acidic environment. It's like heartburn. Burp your baby well, and hold him in an upright position for 20-30 minutes if you can before you put him down. But because he's so young, check with your doctor. Normal growth/gaining is a reassurance.

Crying during sleep: If your baby is fine the rest of the day and night, do not worry. It's surprising but little infants can also dream. If your baby, however, spits up, the crying during sleep could mean acid reflux.