Instead of being awarded flowers when I renovate the lighthouse, flowers are being deducted! What's..

Flowers that you add from any source will decrease the amount of Flowers that you need to reach the next level. So you want that number next to the flower icon to go down to 0 (the bar behind the flower icon will fill up red), which will allow you to level up. Once you reach the next level, the number will then reflect how much more flowers you need for the following level. Nowhere in the game does it show you the total Flowers you have accumulated, only how much more you need to reach the next level.