I am an 18 year old female, 4' 9". I had a period a week ago and ever since I have been having bad back pain, lower right side pain when I lay down. And I get tired really easily. I don't have any STD's, I've been tested, and I don't think it's pregnancy because I just had a period, a longer period then I normally do, and I received a depo shot in Aug if that matters for anything. Any ideas what I could have?