What Causes Delayed Menstruation And Negative Pregnancy Test?

Question : HI Dr. Mintz,

I'm trying to figure out if I'm pregnant. I had my period on 11/11 and it lasted for 5 days. According to my tracking calendar, I should have gotten my period on 12/9. I am on birth control, but I forgot my pill for 3 days after finishing my period on 11/15 (during the week 11/20). I had intercourse on 11/18 and 11/24. Last week I had strep throat and was put on an antibiotic (cephalexin). I did not restart my pill last week because I should have gotten my period on either 12/8 or 12/9. I had intercourse on 12/8 (still no pill). I have not gotten my period as of today 12/13. I took an at home pregnancy test yesterday, and it was negative. I have an 8-month-old who was conceived while I was on the Nuva Ring. Thinking I could be pregnant, but like I said, pregnancy test was negative. Wondering if it was a false negative?