Why does my period predication keep dancing around?

Dr
So I'll log and it will say the 8th, but then when I check back later it says the 16th. Same day, nothing's changed in my log. I don't understand what's happening. Then it'll bounce back to the 8th and vice versa again? 
I would think that if I were logging and it was flexuating that would be understandable but this has nothing to with my logs. It's happens randomly in the middle of they day. 
And that 16th is way off. There's no way it would take me that long to get my period. That's like a 35 days cycle and in a regular 28. So confused!