Sad iPhone

Last night I was talking to my mom on the phone trying to squeeze in the last of our call before my battery died when all of a sudden I couldn't hear her.  I thought the battery had merely died, but in fact, my iPhone decided to reset to factory settings.  WHY? WHY would it do that?!

To top it off, my iCloud has been full for about 19 weeks, and I've been getting reminders that I haven't backed up my phone but the reminder usually comes when I'm not somewhere I can back up my phone, or as I'm going to sleep....so...if anything has happened since June, I probably don't have a record of it.   

Did you back up your data? "Umm I don't do that"