Why am I scared of this?

For as long as I can remember I've always been afraid to use the restroom if anyone can hear. I am in the 11th grade now, but once when I was in 6th grade I was peeing in a school restroom and I heard a girl outside of the bathroom say, "wow, it sounds like a waterfall in there." Since then everytime I pee, I try my best for people not to hear me. If I'm school I will turn the dryer on first or make noise with the toilet paper roller or even flushh while I pee so people cant hear. And if i am at home , I just turn the sink on or tub as I use the restroom. I really need to stop this I can keep doing this forever. What can i do, and does this happen only to me?