[Georgia/USA] I got a bill for a drug test even though I already paid for it?

This past week, I went to an Acute Care facility to get a urine drug screening; I needed the results for a CNA training course I'm about to participate in. The facility didn't take insurance, so I ended up having to pay 45$ out of pocket for them to take my sample. I paid it, got my sample taken and, a few days later, I picked up the results from Acute Care.

Today, I received a bill from the hospital that tested my results. The bill says I owe them - the hospital - 45$. On a side note, the Acute Care facility sent my sample to an affiliated hospital to have it tested.

So, I suppose my question is this: why did I pay Acute Care 45$ for them to simply take my urine, and why do I have to pay 45$ to the hospital as well?

I'm not complaining about the price or anything. I'm genuinely confused about this.