Am I pregnant?

Am
My period is usually pretty regular. I have a 28 day cycle and I always know when I am going to start. However, last month I was 16 days later before I got my period. I had taken four pregnancy tests and all came back negative. This is not normal for me at all. This month, my period is 12 days late so far with no signs of my period coming. I have not taken a test yet because last time I got all negatives. I have no symptoms of being pregnant. Anyone else go through this? 
  Nothing in my life has changed. No stress or anything. A little worried because we weren't planning on having more kids for another few years. We already have twin boys that are almost 1 1/2.
**Updated** I did a home made bleach pregnancy test and from what I've seen online, its positive... should I buy a pregnant test to confirm?