Definitely give her a day or two to respond. She has a busy life and if she responds tomorrow, that doesn't mean she's not interested. It just means that the two of you have had two great dates and she hasn't responded for a request for a third date.
Also, if she's really sick -- like if she has the flu -- she may not be up for going out for a few weeks. So, for now, hang back and let her respond in her own time. It's also a way for you to gauge how interested she is. Sometimes people don't respond right away because they're busy. Other times, however, it's because they're not sure they want to go on that date you've asked about. The pause in that case, may be an indication of her interest, so if she doesn't text back in a few days, you might want to realize she's not that interested. Bottom line: Give this all some time and if you don't hear from her tomorrow, wait a week and then try her once more. Ask her how she's feeling and if she's better. If she doesn't respond to that request, either, then move on.
April Masini is a relationship and etiquette expert and popular media resource — author of four relationship advice books, the 'Ask April' advice column and the #1 free relationship advice forum where over 27,000 questions have been asked and answered, personally, by April. She has nearly a quarter million active forum members, 620,000 Facebook fans and over 1.4 million Twitter followers.