20 month old suddenly hysterical over teeth brushing?

Age 1. Current recommendation is that children see Dentist @ Age 1 for 1st Exam. If you have done this Dentist or Pediatric Dental Specialist has created a baseline. Reexamination can reveal deviation from the baseline. Please call for evaluation.
Changes. Toddlers frequently get upset over transitions, even ones that they have had before. Pick your battles. Offer her the tooth brush and make it a game. Talk about it when it isn't an issue. Give her choices where you can- let her choose what color toothbrush.. Avoid making it into a big fight- if nothing else works, wipe her teeth with a washcloth until she gets through this.
What changed? I would consider any changes that might have happened: teething can cause discomfort and testiness, did you change tooth paste? Some toothpastes have a burning sensation for some. Change toothbrushes? Stay with a very soft bristle brush. Did you bump a sore area with the brush, causing a reaction with subsequent brushings? Until they settle down, you might try using a wash cloth.