There's a bad sickness going around the prison, fatal in most cases. All the sick people are locked in cell block D, occasionally having someone in to come and kill anyone that died and turned into a walker.

Beth, to take care of Judith, was staying locked in a separate room in the prison for safety.

In fear of being isolated, you told no one when you started getting sick. It wasn't too bad yet, but just in case, you decided to go talk to Beth–just incase it'll be the last time.