What term do you use for an impulsive reaction?

I keep thinking about it and I can't remember a certain phrase. It's not 'whim' but more like,

I didn't mean to laugh it's just my _______ reaction

or

I didn't mean to hit you it was just my _____ reaction.

I think it's "something" reaction, I just can't really remember it and it's bothering me.

Hope I'm explaining myself right. Thanks

Solution 1:

It's just my kneejerk reaction perhaps.

Informal. reacting according to a certain habitual manner; unthinking: a knee-jerk liberal.

Synonyms: involuntary, unthinking, spontaneous.
Perhaps also affective or compulsive.

Solution 2:

gut reaction

See: Collins:

a reaction to a situation derived from a person's instinct and experience

But I admit this is more appropriate to a mental response than a physical one.