From this point on should I just hold out from emailing her again?
I'd likely give the recruiter a call asking for an update as there are more than a few possibilities here that if you are feeling stressed about it, call to get some answers from the recruiter.
What could be happening?
The recruiter may have had some issues either health, personal or family that caused her to be out of the office for a few days. While this is highly unlikely, perhaps aliens came and abducted for the next few months. Perhaps she has gotten a nasty infection and has been in the hospital for a few days. Perhaps someone in her family has had a health emergency and she is away for a while.
Is it possible they found someone else?
Sure, that's possible. You don't state when the internship would start which would be another factor here. Is it starting in June or July so it is still a long time until you'd start or is this something you'd start the day after the offer? There are more than a few pieces to consider here though I really would call the recruiter and see if that gives more details. I can remember waiting for over a couple of weeks for an interview because the manager was sick with a cold.
Keep in mind that you are calling because you want to follow-up as you didn't hear back after the e-mail about what is the next step here. Thus, one call isn't likely to make you seem like a pest. Now, if you called daily for the next 2 weeks then I could see you being a pest. Don't forget that if you get the recruiter's voice mail there may be an explanation there that the recruiter may be away on vacation for this week or something.
I'd suggest waiting if you know of some point in the future that can act as a deadline. "We'll know by the middle of next week," would be a message that tells me to call on Wednesday or Thursday if I don't hear before that point in time. At this point, you don't have any such deadline so you call and ask for an update as well as what can be done for the next steps that may come soon.