What Is A Chemical Pregnancy?

What exactly is a chemical pregnancy and how do i know if that s what happened to me this month? i had: sore breasts for 2 weeks very hungry extremely fatigued nauseous vivid dreams cold/runny nose sensitivity to all smells short fuse...my poor husband when i finally did get my period it was a few days early and very, very light. i took a test and it was negative. i did have a miscarriage in january and since then my cycle has been all kinds of crazy.