I'll bet it's Google+ Photos notifying you that it's been 3 days since it was last able to back up photos. Open Google+, tap the Menu button, then Settings. Now tap Auto Backup. Is it turned on? Look at the switch at the upper right.
If it's on, then look under Backup Settings. There are various settings for when photos and/or videos will be backed up (e.g., only when on wi-fi). It may be that your phone is set to backup only while on wi-fi, and you haven't been on a wi-fi network for 3 days. So Auto Backup hasn't been able to backup for 3 days, and is warning you about that.