It's not A's interruption that makes you feel bad, it's your interpretation of it that makes you feel bad. And your interpretation is entirely under your control.
Your caring nature may indicate that you need validations from others, and A's behavior devalues you. It's that feeling of being devalued that is bothering you.
The need for validation is a reflex conditioned since childhood, very hard to change. But knowing the reason might help alleviate that bad feeling.
You want to act like you are not easily bothered, especially in an office.
People behave because who they are, not because who you are.
