Hi there Holly and welcome
Can you please hold the home and power buttons for about 10 seconds until you see the apple logo, then release.
Let the phone restart and tell me if this corrects the problem for you.
It is not a common issue so I want to try to rule out the causes for this on your phone.
Please let me know the results.
I've done that many times. I just did the restart then put in an event for tomorrow.
I put in Alert: Two hours before
Second Alert : 1 hour before
When I hit Done, it switched them.
Then when I tried to edit it, it made bot alerts, first and second the same. so they both said 1 hour before. Or the both said 2 hours before.
Maybe I should take phone to Apple store. I've only had the phone a few months and it has lots of open memory. Not that that should be part of this problem.
Are you adding events to the inbuilt calendar or an online one such as Gmail or Hotmail?