20 weeks, Pelvic Rest!! (Long post)

So I just had my 20 weeks sonogram & my baby is super healthy and everything came back fine. My doctor just called me to let me know and then told me I was on pelvic rest until my next check up. I was to shocked/scared to think of any questions, she just stated no sex, heavy lifting, or lots of walking. I will call tomorrow with the questions I have since she left for the day. But does anyone know the causes and outcomes from this??