Why would auto insurance company continue to work on claim if I signed a full release of liability?

I accidentally signed a full release of liability only 3 days after auto accident as I was misled and on Vicodin at the time. I was to only get $800 'I'm sorry' money and $1500 in medical to be used within 45 days. It's now month 3, almost 4 and they seem to have reopened my claim and are evaluating medical records. I have 2 herniated discs in my back due to accident. Have been getting physical therapy at chiropractor ever since and just got my 1st double epidural cortisone shots in my back. So far, shots no good and horrible side effects for over a week now. Do I have any chance for legal action at this point? The claims adjuster said that because my injuries were worse than anticipated, they are evaluating my medical records. Also, accident was not my fault. Other driver ran stop sign and hit me. Truck was totaled and paid out well for that. Thank you