What Causes A Missed Period With Negative Pregnancy Test?

I am 35 years old. I had a tubaligation 5 years ago. MY last cycle was June 25th. I have taken 4 pregnancy tests that have all came back negative. I have had a upset stomach, been very tired and have had light cramping. I do not have any insurance and I am not sure what is wrong, what to do, or how worried I should be.