My lab mix suddenly becomes aggressive?

in early september of 2010, me and my roommate went to the shelter and both got puppies a day after each other b/c our other roommate got a dog from there too.. my dog(as the rescue shelter says) is a lab/pitbull mix, and my roommates is a pitbull mix.(once again, what the shelter says. not sure what exactly they are because shelter obviously doesnt care enough to actually get it tested.) the lab and pit bull are pretty much the same age and have been living together since we got them. they were best friends and always used to hang around each other and sleep together.. my lab mix is a male and the pitbull mix is a female. they are both neutered and spayed. my dog has been around plenty of little puppies and full grown dogs and has been fine. but it all started when both of them had a piece of wood in their mouths and then they just started going at. now, randomly, when i go outside with my dog, even if the pitbull isnt looking he will stiffen up and i know he is about to growl and go after her so i know to hit him on the nose, say no, and bring him inside because if she knows hes doing it she will go after him too. he always starts it though. also he is a very shy dog, scared even. i think he used to be beat b/c he will not leave my side, i dont need a lease b/c hes too scared to run away, and when hes scared he hides behind me(so much for your dog would risk his life for you) im just curious why my lab goes after my roommates dog even though they used to be best of friends.. O also, my other roommates dog, is much older than the pitbull, is not neutered, and my dog gets along fine with him... my dog is well trained by the way.