What Causes Delayed Periods With Negative HPT?

My period got delayed for 1 month and I am having a upset stomach I did a preg test last week, but neg. Then I went to my family doc, and did a blood test. Resalts coming on this thursday. I am little bit worried now. I am 28 years old. My husband and I, we are trying to have a baby since this Jan. I got a 28days surcle, but Jan. I got 5days delayed then Feb. I got 8 days delayed and now almost one month.