"I apologize to the pap for a stunt that was done 4 months ago regarding an umbrella," Spears, 25, writes on her Web site.
"I was preparing my character for a role in a movie where the husband never plays his part so they switch places accidentally.
"I take all my roles very seriously and got a little carried away. Unfortunately I didn't get the part."
The umbrella incident took place during a period of strange behavior by Spears, during which she shaved her head, got two new tattoos and hit the L.A. club scene.
Shortly before she checked into rehab, she bashed a paparazzo's SUV with a green umbrella following a visit to estranged husband Kevin Federline's house.