If they throw you anything out of the blue and you catch it because you have fast reflects you are...
Many options. A few include:
You are very responsive.
You are very alert.
You are very reactive.
Alternatively, a simple word quick can also work!
You throw a stone at her out of the blue, and she'll catch it! She's very quick!
You are looking for the word deft, along with any of its synonyms.
"She deftly answered all the questions thrown at her."