Please help.

My period is 3 days late. I had a low dull pain on my left side in my lower back then the next day just plain old lower back pain, but it now has went away. I tested 2 times both were negative. I am 3 months pp formula feeding. I have had 2 periods since birth my app the second time i got my period was right and dead on time it was supose to come now my 3rd one hasn't came? What could be going on...