What Causes Persistent Numbness In Left Arm?

My left arm has been numb (lightly but consistently) for 40 minutes without any improvement. For the past several days my left arm (and my right, but to a lesser extent) has had this weird feeling like I won t be able to move it if i try. Naturally, whenever I notice it, I try to move it, just to be sure, and every time I test I find there s no loss of movement ability or feeling. Alongside this it has felt a little achy and fatigued despite the fact that I ve done nothing out of the ordinary to exacerbate it. I spoke to a doctor earlier today and she said not to worry about it but to come back again if I felt any numbness or tingling.