My Periods Have Always Been Regular And I Know By

My periods have always been regular and i know by clock that i am due on on the 28th of every month without fail. Last month i was three days late but my period arrived on the 31st day. This month again i am late and today is the 31st day! My lower stomach cramps have been aweful over the last few months but i have had blood tests and done a 24 hour water sample but with know problems. It is slightly worrying as i also use the fertility tests and on the dates that i thought i may be fertile as i have always been right on time, the fertility test came back as negative too!