Well, the internal bleeding indicated by bruising means you need to go to a doctor to get some diagnostic testing to try to determine what is the source of your internal injury, if indeed you can think of no reason why you are now bruised along your back- from an external source. Hopefully they will take some radiological pictures of your back.
You aren't taking any blood thinner medication (anti-coagulants) such as Coumadin, Warfarin, Plavix, or aspirin are you? Cushing's syndrome can have easy bruising as a symptom, but that symptom typically comes along with a number of other symptoms.
You may have a vascular disease, or some type of hemophilia, there's many more exotic diagnosis but they're very unlikely to have gone unnoticed for this long, or even happen at all. Vascular problems are a serious issue and you should see a doctor asap
how old are you, when did you start having the symptoms? are you male, female, do you have other symptoms? have you been checked by a specialist for possible infections?