Most insurance companies define a pre-existing condition as one that you have seen a doctor for within the previous 12 months. Once you get insurance again, you'll need to find out how they define a pre-existing condition.
If your back pain is severe, don't let this stop you from getting treated for your back problems even if you don't have insurance.
If it's mild, most doctors will tell you to take NSAID's like Ibuprofin and prescribe physical therapy to start with. It's when you don't get better after about a month that they will do MRI's and further investigations.
Back pain can be caused by muscle-related problems such as pulled muscles, or by spine-related problems such as a herniated disc. Muscle pain will usually go away after a few weeks with ice/heat therapy and tylenol/ibuprofin. Herniated discs usually need NSAID's and physical therapy and also can improve but need much more time (months or years) to improve, and sometimes require surgery to fix. If you have a herniated disc and don't get treated early enough you can end up with nerve damage sometimes. Herniated discs also often come with the symptom of sciatica.
Best wishes!