Calendar app on iPhone 7plus

Out of nowhere my phone notified me that I have an event coming up using the calendar app. I was getting my kids ready for bed not even using my phone. Then a few minutes later it dings again and says my phone is at risk for viruses and I need to add malware immediately. I was too nervous to open it so I left it alone but now it sends me the same notification every 5 minutes telling me I have 60 minutes then 55 minutes then 50 minute and so on to get protection or my phone is doomed. This is the same calendar app that came equipped with my phone, so what’s happened or happening? I erased the app anyway but I’m still getting those notifications