There's a difference in other cases:
I've been thinking about it:
Estive pensando sobre isso = correct use.
Fui pensando sobre isso = wrong use (unless you mean you were going somewhere and along the way you thought about it).
To Be = Ser ou Estar, u know, but in Portuguese, "ser" means a definitive state meanwhile "estar" means a transitory state.
Ela é irritada. (She's a angry person).
Ela está irritada. (She's is angry at the time)
Sorry if I don't write very well, I'm learning.