Collections and medical?

A few years back I got a letter saying they where going to garnish my check for a medical bill. When I contacted them they said that they had sent out notices and even had one hand delivered to me. I never got any of this. I never even remember going to the doctors at the time they say that I did. The lady on the phone was really rude and treated me like I just did not want to pay. After the call I burst into tears. I was garnished around 150.00 a month for over a year. Seemed there was nothing I could of done about this. How can this happen? How can you be garnished by collections with out even being looked at twice, making sure that the charge is legit?