Am I responsible for the sewer bill when the MAIN SEWER LINE was cleared, not my personal sewer line?

I live in a duplex with an HOA. I encounter a sewer back up problem a month ago, not just my unit, but the unit connected to mine also had a back up. No one occupied the unit at that time so I called the Health Department because there was human feces on the public sidewalk between our duplex and the neighbors duplex. The HOA was not responding to my calls. The Health Department responded quickly and a Sewer & Drain Co. was called out quickly, I did not call that Co. The plumber who came out said that it was NOT my units fault, it was the main sewer line, and that is stated on invoice as well. He did not present me with a bill, but I did sign a work order so he could prove that the work was done and he passed bill on to a property management Co. Now the HOA is circling this bill around to me and saying that I need to pay it. This problem was not due to me, it was the MAIN SEWER LINE. Is this my responsibility?