What Causes Knee And Lower Back Pain With Dark Stools?

Hi! I have been on augmentin for 4 1/2 days for a sinus infection. Starting yesterday morning I have been able to see my heart beat in my left breast. I went to the emergency room and my EKG was normal. (On a side note- when I used the bathroom yesterday my stool was ornage-ish and I have been having knee and lower back pain). What is wrong with me?