general question amortized cost and worst case
The answer hinges on the definition of amortized cost. Since you haven't given such a definition, let me assume that you are using the common definition.

Consider a data structure supporting operations $O_1,\ldots,O_m$. These operations have amortized cost $T_i$ (where $T_i$ could depend on parameters) if the total cost of a sequence of operations run immediately after initializing the data structure consisting of $c_i$ many operations of type $O_i$ is at most
$$ c_1 O_1 + \cdots + c_m O_m. $$
This bound only works if we consider all operations done on the data structure since initialization.

In your case, you are only considering one part of the operations performed on the data structure, and so amortized cost doesn't apply.