What Causes Delay In Menstruation Other Than Pregnancy?

Hello doctor, I am 21 years old and I realized that my period is 5 days late. I have a regular 28 day cycle and my last period started Jan 9th and lasted for 8 days. I am sexually active but always use a condom. I was worried so I took a home pregnancy test and it came out negative. I have no symptoms of pregnancy apart from the bloating. Usually when my period is about to start my breasts get tender but no signs of that either, so I have no idea what is going on. I had an increase in discharge from my vagina but nothing other than that. Also, about 3 weeks ago I had a surgery on my ankle and had been on strong painkillers for 2 weeks, I am currently off painkillers but I was wondering if that could have affected my cycle? Please help I'm worried