The veteran backstop missed 10 games earlier this season with a testicular injury after he was hit by a foul tip in a game on May 30.
"He’s at the hospital," manager Buck Showalter said, according to Eduardo A. Encina of the Baltimore Sun.
"Doctor on duty looked at him and they decided to get some tests. They are conducting those tests right now. He had the same issue before so, (head athletic trainer) Richie (Bancells) and everybody wanted to be real cautious with it."
In his first season with the Orioles, Castillo has hit .293/.330/.505 with 19 home runs and 49 RBIs in 85 games.