What Causes The Symptoms Of Pregnancy After Having The Tubes Tied?

Ok here's the deal. I'm 25 yrs old I have 3 living children, I got my tubes tied Aug 2009, I had light bleeding that isn't the same like my normal periods, I have all the symptoms that I did with my three daughters I haven't done I test yet. I was wondering what the actually likelihood of me becoming pregnant actually is. I don't want to waste money on a test if it's not even possible.