Went to the dentist and she didn't take x-rays, so could she have missed a cavity in between teeth?

Absolutely: In general, i inform my patients that my diagnosis of their oral condition is a roughly 40% clinical exam, 40% radiographic (x-ray) exam, 5% verbal and written history, 5% risk evaluation. Yep, you noticed! that's only 90%! that means i'll never know the other 10%! that's made up of what you ate, bit into, whatever for last year, last decade ; before. If risk is low, annual x-rays aren't needed.