I would contact the hospital's billing/adminstration department and tell them what happened. Explain you are really unhappy about paying $400 for a misdiagnosis that results in a weekend of pain and additional medical bills. Most hospitals are willing to compromise a medical bill if there is a possibility of a mal-practice claim.
Before making a big deal out of this, was the first doctor negligent? Should he have found the problem or was additional workup needed? Did they recommend seeing a specialist and getting further tests? Ordering CT scans when a patient is not insured can be a problem. What if nothing was found? Would you then have been willing to pay the $1200 charge for the CT Scan?
The problem is for $400 it's not worth pursing a claim or getting an attorney. Most hospitals will allow you to make payments over time - say $100 a month for 4 months. Hospital I've found, are usually willing to work with you not against you.