I had sex about 5 days ago, and we used a condom. However, I am not on birth control and am still worried I may be pregnant. The nausea and back pain started right away after we had sex and hasn't gone away. So just in case I took an emergency contraceptive pill on the 4th day which was cleared by a clinician. However is this still a sign of pregnancy?

Sounds more like a stomach virus. At this point, it's only been 5 days, the fertilized egg, if indeed it is fertilized, has not yet implanted into the uterus. You would not be having symptoms this soon if you were pregnant.

Ok, so with all of my chances, using a condom and the emergency contracpetion would you say I am most likely not pregnant?

I honestly can't answer that question. Everything depends on whether you were ovulating and the emergency contraceptive pill works best within the first 72 hours (3 days).