Norwood picked up 57 caps for Northern Ireland but decided a year ago to concentrate on his club career as he prepared for the challenge of the Premier League with the Blades.
"I always found that the first two or three games after an international break I was struggling. You get to a point where something has to give," he told BBC Radio 5Live.
"I noticed a difference in my performance before I went away compared to my performance when I came back.
"It was affecting my performances for whatever club I was playing for and I didn't think I was giving myself the best opportunity to perform for my club.
"It's probably not good for people to hear, but at the end of the day your club career comes first."