Be caught short
Definition 1 of 2
Pro Tip 1/3Urgency
Expresses a sudden, urgent need related to bodily functions. 
Long queues can be problematic if you're caught short.
Pro Tip 2/3Informal
Used mainly in informal situations among friends or family. 
Let's take a break soon; someone might get caught short.
Pro Tip 3/3Embarrassment
Mentioning this phrase could imply or cause embarrassment, use with discretion. 
Sorry for the interruption, I was caught short just now.