If you want to know how far you are from your next level do:
Your level times 50 minus 50
So if I'm level 20
20x50=1000-50=950
I would need 950 exp points.
Once you get his number subtract your current exp points to get the amount you need.
So I need 950 xp points to level up to 21 but i already have 750 so
950-750=200
I would only need 200
The bar may be full but that doesn't mean you cant level up. Its a visual glitch that your bar is full. It doesn't mean you have full exp points.

