Could it be he felt lethargic and all due to Ruyi is pregnant? There are some cases where the father will experience morning sickness instead of their wife. In severe cases, some even fainted and vomiting none stop due to their partner pregnancy. So that's why the doctors couldn't find the fault in his body but instead it's because of this morning sickness that his wife should experience but he's the one who felt it instead?
Interesting theory but he didn't behave this way with any other pregnancy in the harem. At this point the emperor is starting on the decline into debauchery.
