What Causes Delayed Periods After My Husband Had Vasectomy?

I have never been late with my period. My husband had vasectomy 10 years ago (never tested sperm after). I am 8 days late. At the very beginning of my last period I was very, very sick with the flu and strep and I got a steroid injection. I am 40 years old. Three HPT all came up neg. Should I assume my illness screwed up my cycle?