Sharp pain in right shoulder, left side pain, missed period. What is happening?

Ectopic pregnancy. The shoulder pain may be from "air" or fluid under the breathing muscle(diaphragm) irritating the phrenic nerve from a ruptured ovarian cyst or ectopic pregnancy. Get a pregnancy test and examination by a gyn to sort out all the possiblities.
Depends. The most dangerous thing would be a tubal pregnancy. It can cause internal bleeding and can be life threatening. These types of pregnancies usually present with side pain and shoulder blade pain early in the pregnancy. I would recommend being evaluated as soon as possible. If you are not pregnant, it could be an ovarian cyst, or a myriad of other things.