Help! I am being scammed by a doctor's office!

In March of this year, I visited a local doctor’s office to have blood work to check my vitamin levels. A few months later, I received a bill for more blood tests allegedly done six weeks after my initial visit. Although I did not go to the doctor to have a second set of tests done, the doctor’s office is claiming that I was there. At my first visit, I was required to sign a “client bill acknowledgment” form because I don’t have insurance. I asked them for a copy of the form that I would have had to have signed on my alleged visit and they replied that they didn’t have one because they wouldn’t know if it was a patient bill unless I told them it was, therefore I would have had to file it with my insurance company. I replied that I don’t have insurance. At that point, the doctor’s husband, who is an attorney, walked in and informed me that I had the option of changing it to a patient bill, however I would be charged a $50 paperwork fee. The first set of tests included about 20 different areas tested and cost me $152. The second set they are claiming I had only included three vitamin levels, costing me $323.50. I am really at a loss of what to do next. I am absolutely positive that I did not have a second set of tests done. The date they are saying I was there was two days before my 21 birthday. There was no way I could have taken off to go to the doctor, or even afforded to pay for another doctor’s visit, considering I was going all out for my 21 birthday. Help! What do I do?