I got my Fitbit Flex a few months ago and it also showed an arbitrary number of calories burned per day before I even had my Flex (1758 for me). Not sure why it does that, but after having the Flex for more than a week or so, it doesn't really affect my actual data at all.
I wouldn't worry too much about your data from the first day of actually having the Fitbit. Something to keep in mind though, your Fitbit automatically changes over to a new day at midnight and all your data will start at 0 at midnight. 0 calories burned, 0 steps, etc. If you have you Flex on while you sleep, you may end up showing some steps first thing in the morning. You will definitely have some calories burned as the Fitbit takes into account your calories burned just from being alive. That's probably where the 1486 calories burned is coming from. I wake up between 4 am on workdays and 9 or 10 am on days off and my Fitbit will show between 300 and 600 calories burned already depending on when I wake up, just because of calories burned from living in that time period.
As far as I'm aware, there's not a way to reset the data that your Flex has already tracked, although I haven't looked for it so don't quote me on that. That's why you may want to ignore the data from your first day, just because you weren't actually wearing the Flex since midnight when it automatically resets to a new day with everything at zero.
