Oh, sweetie... so sorry to hear you're going through this!
With a miscarriage, you can expect about what it feels like to have your period times 5... you'll have a lot of cramping and bleeding as well as the back pains... difficulty breathing tends to come as a reaction to the pain. Take whatever you would normally take for period pains to ease your discomfort. And DON"T miss your next b/w appt. They need to check that number and make sure it's going down so they can rule out ectopic pregnancy (pregnancy anywhere outside the uterus). An ectopic won't double normally but will continue to increase your HCG levels... and could be life-threatening. Based on your situation and what you've described, I don't believe you are experiencing an ectopic pregnancy but it's always better to be safe than sorry.
Again, so sorry you're facing this right now. It's never an easy situation. Good luck!