Does it sound correct if I say "After the big earthquake, whenever I feel a slightly big quake, I immediately put myself under a table. It's like my knee-jerk reaction." Or should I use some other phrases rather than "knee-jerk reaction" in this case?
Getting under the table, although probably a conditioned response, is mostly a deliberate one (you might use "immediate", "initial" response; or, although you did probably give it a nanoseconds' thought, "without thinking I...").
Knee-jerk reactions, are more reactive, less deliberate, and equally prone to conditioning but more toward the edge of irrational predispositions of personality. Knee-jerk reactions are frequently emotional, confrontational, and less mediated by an end consequence (i.e. surviving the earthquake).
If you're interested in using an idiomatic phrase, this might work better:
The big earthquake made me gun-shy. Now, whenever I feel a slightly big quake, I immediately put myself under a table.