While extremely unlikely, it is technically possible.  Since the cards are designed to be read from up to 10 centimeters away, a criminal would have to get extremely close to you with a reader.  But remember, the issuer of your card and the payment brands have put security measures in place to protect you, so even in the unlikely event that a criminal has read something off your contactless card, he or she can't do anything with it.

For more information, visit the Smart Card Alliance website