sharp lower back pain every so often? I'm 18. Thanks?

[lalallala]

I was two weeks late when I started my period, then after five days of brown/red bleeding it was gone. Since my period ended I have been having lower back pain and its sometimes one sided. I've just been experiencing unusual, sometimes painful feelings in my lower abdomen? I really thought I was pregnant but now I've had my period I know I am not. About a week after my period ended I went to the doctor who felt my tummy and said ' I don't think I feel a baby in there'....I don't know why I am cramping though...Please help. Thanks