Baby went floppy for a minute?

My daughter is 5 months old. I was in another room folding clothes and left my daughter on my bed (it's a double bed with things on either side so she can't roll out). She was having a little bit of a moan, but then I heard her start accidentally tapping on my iPad and it made noise so she was happy. When I finished I went back in the bedroom, the iPad was off now and she was lying on her belly (she sometimes rolls on her belly when she's on the bed, but by accident), with her face turned away from me so I couldn't see it, not moving.
I called her name loudly a few times from the door and she didn't response at all and I freaked out thinking I was gonna roll her over and she'd stopped breathing or something. I rolled her over and she was awake but she was limp, like her arms and head were floppy, but only for a few seconds. I picked her arm up and it flopped back down, and then she was fine and normal again and smiling.
Why did this happen? It only lasted a few seconds, it is too late to call a doctor today but it scared the crap out of me!

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