Saddle sore
Definition 1 of 1
Pro Tip 1/3Physical Discomfort
Use 'saddle sore' to express physical discomfort from long periods sitting on a bike or horse. 
She was feeling saddle sore after her first cycling marathon.
Pro Tip 2/3Activity Specific
'Saddle sore' refers to pain from activities involving sitting on a saddle, not general discomfort. 
He wasn't used to the bike yet and became saddle sore quite quickly.
Pro Tip 3/3Literal vs. Figurative
While often literal, 'saddle sore' can be used humorously to describe other uncomfortable situations. 
Hours in the crowded bus left me feeling saddle sore, even without an actual saddle.