I am 21 years old female

My mum is a very decent educated single mother but never mentioned our dad(s) at all.... 

In my birth certificate there is no dad's name but in my small brother's there is a name.

I really wanna know my dad and the reason for me not having the name though we've both never seen even the holder of the name in my brother's birth certificate.

Last year sometime I wrote her a message politely asking on our fathers whereabouts but she snobbed it... I don't have the courage to face her face to face but it kills me every day trying to figure things out... yet we never talk about anything with her though we have a very good bond and we get along quite well..am even lacking sleep all night coz my head is all-over

Sometimes since I've known myself she recieves calls from a number that she keeps on changing the name, but she always goes out to answer or answers it weirdly and I have my suspicions that it might be one of them

I really need some advice on what I can do.I would like to know whether am being ungrateful by wanting to know my dad.. And I also wonder why my brother would have a name but me I don't