Colour change of fluorescein in acidic and basic medium
The lactone form is not stable in basic medium, so the cycle opens and gives conjugation to flow and unite all four rings. The problem is you really swithched the acidic and basic forms of fluorescein, so the left one exists at basic pHs (except for the H atom at carboxylate - it goes away), and acid medium favors the existence of the right form. 
You may check the forms of phenolphthaleine - they are pretty much the same.