What’s your mother tongue?

I was born in Bangalore and I spent my early years in Bangalore and Chennai where the languages spoken are Kannada and Tamil Respectively.
My mother’s native Language is Hindi and Father’s Tamil.
At home we speak in English, so my dominant language is English.
I moved to my mom’s native town where Hindi is spoken.
So the question is what’s my mother tongue?
Though I can speak in English as now I am learning to speak in Hindi, I believe my mother tongue is English because English is my first language and this is the language I use to communicate with my mom.
Isn’t mother tongue the language in which you communicate with your mom, Well I understand it like that. Once in Pre-KG my school teacher gave a comment about me that I was not good in my mother tongue and I wondered if she even knew what my mother tongue was. Just staying in a city does not make the local language of that place my mother tongue. Does it??
English is the language I heard after I was born and English is the language we used to communicate at home, so I consider English as my mother tongue.What’s your mother tongue?