Suggest Remedy For Pain In My Upper Back

Hi, my upper back/shoulder on the left side hurts immensely. It feels as if I had been stabbed. I did not injure myself, and neither had experienced anything like this before. I am only 21 years old, female, so I am clueless as to what it is and how should I treat it. It started hurting last night, little by little, and when I woke up today I could barely get up from bed...