I need advice

I am 5 days late, I had a period last month and the month before and haven't completely missed a period in awhile, but nothing this month. I had surgery on the 15th and they sampled my urine so I figured I wasn't pregnant but maybe it was too early to tell? I feel like my calendar may be off. I'm just confused why I wouldn't have a period at all this month. I am not on birth control.