Shorter wake windows?

Hi all. My 5 month old boy usually has a wake window of around 2 hours before each nap, however these last couple days he seems to get very tired and sleepy after 1 hour into his wake window. He gets around 10-11 hrs of nighttime sleep so I don’t get why he’s suddenly so tired during the day? I wouldn’t have thought that his wake windows would get shorter. Anyone has any advice as to what could be causing this? Xx