Ok everyone who may be reading this now...PLEASE get a CT scan and check for pulmonary emboli and/or pulmonary infarct! I had these EXACT symptoms last week, and after initially being diagnosed with pneumonia (my vitals were good, oxygen levels good, x ray doesn't show blood clots, etc...) I had a hunch something more serious was wrong. Thank God my PCP recognized my symptoms immediately and sent me for a scan which revealed numerous clots in my lungs along with an infarct. Anyone who has these symptoms most likely has a clot that moved into the lungs and is now blocking off blood supply. The longer you wait, the more damage will be done to your lungs and the more risk of death you will have. It feels like a pulled muscle at first along the back in almost all cases, and many people recall not doing any major movements prior to the "pull." Then, it progresses from there into sharp, stabbing pains that can come and go periodically depending on position. Laying down on your back or stomach usually makes it worse. Breathing deeply hurts. Ribs can begin to feel like they are almost "popping" in and out. If you have any of these or all of these symptoms, DO NOT WAIT and do not leave the ER until you have a CT scan of your lungs done! Even if you are low risk for clots. I am perfectly healthy, fit, active 33 year old female with no smoking in my history and only birth control years ago. Do not think that this couldn't happen to you, and don't listen to any doctor who doesn't at least test for clots! This could save your life! A similar story is what saved mine and made me go back for a second opinion. Please take this as your sign to go get checked now! (also note that everyone's symptoms can vary slightly, but most certainly if you have these specific symptoms, do not hesitate, go NOW to the ER!)