Why did my friend ( female) stare at my bulge when we were in the hot tub?

We were in the hot tub on my back porch with some friends and joking around as usual and when we got out to cover the tub back up while two of my friend were showering i was laying down to latch the top down. My suit was wet so I had a bulge and I looked at her and she kept looking at it for like 10 seconds. She blushed then smiled and helped me up and we got cleaned up and started drinking and watched a movie as a group. I can't get that moment out of my head.
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Why did my friend ( female) stare at my bulge when we were in the hot tub?
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