A non-graphic story about giving blood

Today I gave blood, and immediately after being stuck, the nurse said, "Whoa. WHOA. wow! Did you feel that?"

I've given about two gallons of blood in my life... enough so I wasn't concerned going in, but I was freaking out after the first Whoa. Any  physical sensation bottlenecked with panic.

"No," I said, trying to concentrate to see if I felt anything unusual. I didn't.

She said, "The needle just completely turned around."

I just looked at her so she explained further. "Sometimes we just let the needle do what it wants after it goes in, and it just turned completely around. Wow!"

I did want her to elaborate, but I couldn't bear any gross image during the actual giving blood process, so I decided to ask after it ended. Unfortunately, she wasn't there when I finished, so I could never ask.

But, like, whoa! What happened?!?