I am a swimmer and i tripped over my dog about a month ago and i fell on my left shoulder. it has been hurting for a month, what could it be?

Could be: A ligamentous injury, fracture, muscle pull, tendon rupture. Rotator cuff injury is pretty high on the list, but you need an evaluation by someone who knows shoulders. Best to start with your primary doctor. If they can't figure it out, they'll send you to an orthopedic surgeon.

Lots: A couple of possibilities. You may have separated it, or damaged the muscles (rotator cuff). I would work on range of motion exercises and if it continued i would see your doctor.

Many Possibilities.: This could be shoulder, neck &/or brachial plexus problems. A month out, fair chance you have tears or fractures which remain out of position and are not healing. Look for excellence in assistance from a physical therapist first. May need assistance from an orthopedic surgeon, hope not, but certainly possible. Of course surgery involves additional, more controlled injury & does not always solve.

Shoulder pain : Traumatic shoulder pain need to examined for subtle fractures and also tendon and bursa lesion an proper exam with imaging is going to give us the right info to stablish the proper diagnoses.