swollen dog lymphnodes

hi! from my understanding of the lymphatic system, your lab may be suffering from an infection or inflammation. if the swell is under the armpit, it could perhaps be a wound around the upper body (torso) or a muscle/ joint pains anywhere from the neck down to the back and front just before the groin area. if it is a lymphnode, the swell is actually a way for the body's immune system to begin the process of healing an infection or inflammation, it's the white blood cells working overtime. the larger the swell, the larger the infection or inflammation. in some human cases, when a person is overworked and too tired and kinda pulled a muscle on the back and arms, the armpit can swell too. if you can, try to feel around his body for any trace of wound or inflammation. and try to walk thru the things he did prior to the swelling. it can be helpful when you go visit the vet soon. if the swelling lasted for more than 2 days, it means your lab's body cannot cope with his pain/infection by his body's system alone and he needs treatment and medication. hope your dog gets well soon.