Hi Jenny Housh!
Thank you for sharing your question with the EmpowHER community!
I am sorry to hear that you are under a lot of stress lately! You could be feeling some numbness in your left arm for many different reasons. Most likely the culprit is that you are not getting enough blood supply to your extremities. Other explanations could be nerve damage or a pulled muscle, which could explain the pain in your shoulder. It could also be the sign of something more serious and therefore I don't think it should be ignored.
You are really young and it sounds like you lead a healthy lifestyle and heart attacks aren't as common at your age. However, since you explained that you have a history of heart problems, I would definitely seek your doctor's expert opinion. From the research that I did, typically stress isn't the cause of what you are feeling.
Here is a link form the EmpowHER community about signs of a heart attack.
Jenny Housh, are you able to schedule an appointment with your doctor fairly soon? I don't think you should put these symptoms on the back burner. Let me know.