I too suffer from migraines, and have done pretty extensive research on the topic. Turns out, migraines are a type of seizure disorder, but only about 10% of migraineurs have motor manifestations; usually it only disrupts electrical impulses in neurons modulating pain. Sometimes this is called a Basilar Migraine because of the part of the brain where the electrical activity begins. Do you usually get the headache on the same side or does it change? Either way, your symptoms are consistent with prodrome for this type of migraine. I get motor symptoms sometimes too, but instead of prodrome, I get muscle tremors during the headache itself or during aura. But the bottom line is it's a good way to know the headache is on its way and possibly be averted. I would recommend you ask your neurologist about the symptoms, especially if it is a new symptom. Good luck!
Welcome to the MedHelp forum!
It could just be an aura of migraine—a symptom you suffer from just before you are about to get migraine. Otherwise left shoulder pain can be due to abdominal causes such as gastritis, acidity, gastric reflux, hiatal hernia, pancreatitis, or local causes such as arthritis, rotator cuff injury, bursitis etc. You should get these causes evaluated. If they are ruled out, then it is an aura of migraine.
Do discuss this with your doctor and get yourself examined. Take care!