Learn English – What does “Here it comes” mean

“There’s something I need to say,” Riley says at last, her voice
serious…

Here it comes, Gwen thinks.

Could you please tell me what the meaning of "Here it comes" is?

The fuller text:

Maybe it’s time to try. Maybe it’s time to let go of the baggage, the
guilt, and try to live her life, she thinks. Maybe last night is a new
beginning for her. She feels a surging warmth and happiness inside
about David that she can’t help, even though Dana is dead. She’d
wanted so much to go to him just now. But it would have been
completely inappropriate. They’d managed a warm glance at one another,
but that was it. There was time. They would be together again. Riley
won’t like it that she was with David last night. Gwen knows that, but
Riley is her friend, not her keeper. Riley should be happy for her
that she’s met someone. Gwen was always happy for Riley when she met
someone, and Gwen usually didn’t have anyone special herself. She’s
sorry it had to happen when they were supposed to be spending time
together this weekend, but you must take good things when and where
you find them. They are rare enough. Dana’s dreadful accident has
brought this home to her. Riley should understand that. It’s not like
she planned it this way. They reach the room and Riley closes the door
behind her. Gwen looks up at her warily, waiting for her to say
something. When she doesn’t, Gwen reaches for some clothes from her
overnight bag. She would like a shower, but that seems out of the
question. The water will be freezing. “There’s something I need to
say,” Riley says at last, her voice serious, as she pulls a top on
over her head and flips her long hair over her shoulders. Here it
comes, Gwen thinks. “That attorney, David Paley.” “What about him?”
Gwen’s voice comes out more sharply than she intended. “Did you sleep
with him?” “Actually, yes, I did.”

An Unwanted Guest By Shari Lopena