Still in so much pain and not on my monthly anymore

I'm 20 years old just finished my monthly for this month and I'm still aching and my right side of my body still really hurts?? I can still move my right arm and hand but when I went to stand up from my knee down on my right leg was swollen and stiff for a while? I started freaking out! I don't know if I should go see a doctor or go to the ER if it happens again. Has this happened to anyone else before??