Should she say that she is pregnant or should she keep it quiet for awhile before tellin him.

It depends on how you want the story to progress. Is it more interesting to you to present how she copes with the anger and resentment in silence, or with the scorn and shame of such a public humiliation? Do you want to focus on how such a betrayal dashes one's expectations and leads to distrust of others in her life as well as possibly acting out in other ways, or do you want her to repress in a different way by presenting a nonchalant reaction, admitting that she's pregnant after that bombshell?

Are you interested in exploring how they would work on figuring out what to do with the pregnancy, or is it more interesting to have her provide some silly, spur-of-the-moment thing that leads him to wonder what she was really going to say? What kind of a reaction was he trying to get by such a public declaration of infidelity - was he hoping to get back at her for something, or to try (in a very broken way) to get her to commit to their relationship, or is he depressed and feels like he can't talk to her about his feelings? The answers to all of these questions should inform your decision.

Well, I suggest that she doesn't tell him. The depression could help her not say anything about her having his child.
But I also want to tell you that whatever you feel should happen is the perfect way a story should go, because you're the author. No one else can say otherwise :)

That is a very interesting question. It will determine where the story goes from there and possibly how it ends. If he's gunna be a big part of how she deals with her pregnancy, then she will have to tell him, probably closer to the start of it, but if he's not, then it could go either way. since she is feeling so depressed though, it might make more sense for her not to, at least at first. I think you should have her struggle with the deciscion for a while, show her internal debate, and decide with her. That's what I did with my book when I couldn't decide whether or not my main character would commit the murder of her best friend or not.

Make it to were its on the tip of her tongue then at the last minute she changes her mind.

Totally she should tell him!
Boy: I made out with another girl
Girl: Yeah, well while u were doing that I was dealing with being pregnant!