Aggressive choc lab puppy

Question
We have a 4 month old chocolate lab puppy and she gets extremely aggresive over food. Shes as friendly as any other dog but as soon as anyone has food she has to have it too. If she gets hold of something you drop on the floor there is no way you can get it back for example, when trying to take her bowl away after she finishes eating she growls and will lunge at you and bite and has biten me severely on many occasions. There is just no way of getting near her to try and teach her otherwise. She has also been having some bad diarrhoea and ive noticed some blood in it and im getting very concerned. The way she acts over food just seems so out of character where labradors are concerned, ive never known a lab to get so aggresive and agressive enough to seriously harm people.  

Answer
The diarrhea and blood call for immediate attention from a vet.  It is possible clearing up a physical problem will help the behavior, but likely it is a seperate issue.  

Being keeper of the kibble is a powerful position.  She sits until you say free dog after placing the dish on the floor.  If she moves, the dish goes back up until she sits.  It will not be easy at first.  If it takes 15 minutes, it takes 15 minutes.  Once you have that working, only put half the food in the dish.  A minute later when she finishes, again make her sit while you dump the other half of the food in her dish.  It will take a while, but as she associates your hand at the dish with more food, she will no longer be aggressive.  

This also suggests a lack of leadership on your part.  Labs can be quick to fill a leadership vacuum.  Dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog.  Life is much easier if the 2 legged pack members outrank the 4 legged ones.  You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. Start at http://www.dogsbestfriend.com/  For more on being top dog, see http://www.dogbreedinfo.com./topdogrules.htm