What does it show on your iPhone under Settings, Mail, Contacts, Calender, Default Alert Times for Events and All Day Events? Sometimes it gets messed up. When this happened to me, I turned off all the alerts and exited and then went back and set Events to 15 minutes and all day to 1 day before.
Also, if you use another calendar program other than Apple Calendar, it could add its own reminder.
I know it doesn't make sense, but the first one fixed my problem. I also was using Fantastical and it had its own reminder so I was getting too many alerts I fixed.
Hope it works for you!