porque sea una gran seguidora

Hola Ελισάβετ

Here in this sentence we could have used the indicative or the subjunctive -both would have been correct grammatically.

We chose the subjunctive to indicate that that bit of information is something either already known or some background information that we don't need to present as something new; so, this is what our sentence in the reader conveys here:

Si estoy tan nerviosa, no es porque sea una gran seguidora de estos premios,...

However, we could have also used the indicative here saying:

Si estoy tan nerviosa, no es porque soy una gran seguidora de estos premios,..

Here, the intention of the speaker is to highlight that bit of information and somehow present it as something new to the listener -this is when we use the indicative.

I hope this helped.

Inma