My puppy eats stones and gr every time we take her in the garden?

our puppy is a very playful little monkey and we have just started taking the puppy into the garden so she can run around and play ball with me which she love even brings the ball back to me. but if im not playing with her ie hanging washing or sitting down, she will eat gr (she is never sick) or small stones. she eat normally and drinks fine, she doesnt really chew my house or other things and she really enjoys chewing her toys. if i try to take the pebbles or gr she will become a bit growly and bark at me. i have been told this might be a sign off something really bad help ....