Skyla

I just got my skyla. Wasn't the easiest-but also not the worst pain. SO my dr said to use a back up for 3 ish weeks... To make sure it was line up correctly after an ultra sound..... And I'm just like wait like condoms or can I just continue my pills? Most of my friends waited a week and then stopped all other BC and never had to have a follow up. Skyla is suppose to start being effective immediately right? I really don't want to pay a $100+ co pay for an ultra sound but CLEARLY. I do not want kids either.