I am having pain under my left shoulder blade and kind of numbness in my arm. Could it just be muscle?

Possibly. These are symptoms that could be potentially serious. Yous should discuss these with your physician. He/she will thoroughly evaluate you and let you know what's going on.
No. Pain under the shoulder blade and arm numbness may mean something going on in your lower cervical spine (neck bone). Neck x-ray would be helpful.

Related Questions

I start getting random pains behind my left shoulder blade, and then my entire left arm gets numb. Heart or Maybe muscle related?

Most likely.. In your age group there is a very low chance of having heart disease unless you suffer of illnesses that pose a risk for this problem. If you are otherwise healthy then this is most likely muscle related and the numbness in your arm could be related to a nerve irritation at the level of the neck or the arm/hand itself. You need evaluation for diagnosis. Follow up with your doctor. Read more...