Yes, Condoms are 99% effective as long as they are used correctly and are free of defects, but there is still that 1% -It is always best to use a second from of birth control, spermicidal lubricants etc.
There's a saying: Sometimes Its a "grower" not a "shower". Besides It really doesnt matter at this point as others have pointed out you are not done growing yet. And Its what you do with it that counts!
If you hear a metallic scraping sound when you brake in addition to shaking you could need new disks (brakes) because they are warped. If it only shakes while moving and you don't hear anything funny your tires could be miss aligned/un balanced -uneven tire wear can cause shaking feeling.
Scabies cannot live without a human host for any more than 36 hours so they tend to stick to people, if your nervous about it don't use your couch for a day or two.