She needs time to think?

Hi, i have fallen in love with my sisters best friend however now that we are going out, she told me she needs time to think! It was so sudden and we were getting on great, and then she just turns around and tells me she needs time? im so upset and i don't really know what i should do because i want to talk to her but i dont want to be annoying and pester her in fear of drving her away?