What Causes Delayed Period After Having Protected Sex?

Hi, I never had irregular period in my life. I m 41 yrs. old, and my family doctor said I have polyp which needed to observe, and I m due for ultrasound. I didn t have my period this month, and it s over 5 days now. I m sure that I cannot be pregnant because my husband use condoms always and very careful. So, what can be the reason for delaying period? Oh, I was sick last week and I took tylenol for some times.