playing together with another dog then my staffy got aggressive

our dog alfie was playing with another dog happily for a while in our local dog exercise area and then the other dog started humping mine and wouldnt leave him alone and alfie pinned him down and was barking and growling at him.
nomally alfie just sits there and lets them get on with it,
i dont know if he would have bitten him as alfie had a ball in his mouth and he has only just started telling other dogs off when he has had enough
he is 23 months old and now i am worried that he is turning nasty with other dogs which is really out of character

He was just letting the other dog know where he stood , if he wanted to hurt him he would of , just take it easy with him around the more dominant dogs at the moment and see how he goes on

thank you, he is normally really good with dogs

Have a read of the link below which gives you the signs to watch for:

http://staffy-bull-terrier.com/is_my_dog_aggressive

sounds like he was just telling the other dog where to go, dog shouldn't have humped him lol