Why did my dog attack my other dog they are not aggressive?

Last night my 3year old Lab/ Blue Heeler mix attacked my 1 year old Chihuahua. They were outside for like 20 mins and then I heard yelping so I ran out there and they both had blood all over themselves. They weren't injured badly, but what could have caused this to happen? We have had no problems with them before this they are buddies always playing together and sleeping together.They are both females and spayed. I also noticed today that my older dog keeps sniffing my younger one especially her private area.

There is a pretty hefty size difference there. It wouldn't take much for normal play fighting to turn into an accidental bite.Otherwise, they could have encountered prey, or some other object which they got in a dominance struggle over.

sometimes its territorial or the dog said something mean

they could have both found something to eat so they had a fight over the food, or because of the size difference it could of been a play fight gone wrong, nothing to worry about

Did you actually see them attack each other? If you didn't actually see it they could have been attacked by another dog/animal.