16 days late, any ideals>?

At my last period I was 8 days late. I am usally on a 30-33 day cycle. So the day before I got my peroid march 30, i had blood test, it came back neg. Now again, this time i am 16 days late this time, hpt neg. having headaches, somewhat lower back ache, feeling a little sickly during afternoon to evening but no vomitting. some light cramping every now and then but no spotting or anything like that. Also I had a gastric byp for weight loss 2.5yrs ago and weight is stable. Any ideals?