Possible miscarriage??

Is it possible to be pregnant but never show up on a pregnancy test and then miscarry?  I'm 3 and a half weeks  late with no positive tests but I feel terrible today. Dizzy, shaky, worsening cramps and started bleeding. The bleeding isn't heavy yet it just started a little while ago. Just had a few severe pains in my back too. Could this just be my period being strong because its late? I've never felt like this before.