What Causes Pain In My Lower Back?

Hi, I am an 18 year old female and I have been having lower back pains for about 3 weeks. They have been getting worse everyday. They hurt when I wake up and am sitting down the most. When I bend over, I feel like my back locks up and I cannot move right away. I have been talking Tylenol and doing some back exercises, but it doesn't seem to get any better. Today I am having some abnormal vaginal bleeding (a very bright red color). The pain is making me a little nauseas. What could this be?