Is a volunteer responsible for a nonprofits bill?

I volunteered from early 2008 to late 2010 at a 501c3. They helped disable and homeless Veterans, homeless and disabled. My husband and I donated money, supplies, food and time. Many Vets have substance abuse problems and are mandated to get drug testing. The nonprofit also provided that for a very low cost. Sometimes when the nonprofit was out of money temporarily, my husband and I paid some of its bills to keep the doors open, including the lab bill.

The last 6 months or so I discovered that the director was mismanaging the funds. I spoke to several board members and was told they would handle it. During that time I stopped financially contributing, but kept donating my time. There were severeal times the director said she had cash to pay the lab bill but didn't dare put it in the nonprofits checking acct because it was overdrawn and asked me to write a check to pay it if she gave me the cash. Since she was giving me the cash to cover the check I agreed. I left about a year ago and later found out the nonprofit went out of business and was evicted.

Last week I was contacted by an attorney for the lab saying that I owed them almost $600 for the nonprofits outstanding bill. I explained what had happened but they said that no volunteer would do what I did and are turning me over to collection. Can they do that? I was not on the Board of Directors, couldn't sign legal documents or anything like that. I coordinated volunteers and assisted people...I was just a volunteer!