Low back pain can be cause by several different problems. If I was your doctor
, I would first ask you if you noticed the pain after an injury to your back, or after a day of heavy lifting. The low back has many muscles that can become pulled or strained causing pain in that area for some time. Low back strains are often best treated by staying in motion and avoiding bed rest.
Other common causes of low back pain include spinal stenosis, herniated discs, and degenerative disc disease. These cause of low back pain typically don't cause problems in women your age, though it is definitely not unheard of. The fact that you feel bruised there with a possible fluid collection suggest to me that you sustained an injury of some type. If this is the case, then with the proper treatment you should get better.
Before taking any medications, or trying any exercises, I would suggest that you schedule an appointment with your primary care physician
. He or she can get more detailed information about the circumstances surrounding the development of this back pain and perform a thorough physical exam. If necessary, your doctor may order imaging such as a low back x-ray or an MRI
. If your doctor thinks is this a muscular problem, then you may get a referral to physical therapy. Good luck.