Vomiting at night while nursing

The past two nights during our bedtime feeding my 4 month old projectile vomited all his milk up. Yesterday I thought it could be because he got his shots and woke up from his 2 pm nap screaming for an hour. We gave him Tylenol assuming he wasn’t feeling good from the shots. He seemed fine the rest of the day until he puked but he was still smiling. Then tonight same thing puked before bed. Today he was fine all day. This ever happen to anyone? Any idea what it could be? Should I be worried? He didn’t cry or seem upset either time.