I was at work and the following happended. What should I think of this situation?

I am working in the footwear department. A customer ask a question and whether or not I can help her. Shes says that there is not one in the outdoor department. She wants to buy a canopy she saw in our advertisement . Our sales paper says buy canopy receive 4 free fold able chairs. I want to purchase this if I can get the chairs. So I go in the back to get a dolly to carry the box. I wheel it to customer service to ring it up. She tells the cashier how there was one there to help her. So she had to get me from footwear. My manager and other employs ask why I took out the dolly . That laugh at me and put me on the spot. I noticed the box had wheels at the both and a handle. Was I wrong for bring out the dolly?How I was very helpful to to her. Shes say why is everyone standing here seems like he's the only one working here. My coworkers and manager where shocked and didn't say anything. Every time I come around they stop talking or they tell me go help a customer. What going on