Here is my advice...

From what you described, that bi^ch was either intolerably stupid (didn't read your face expression after what she said) or did it evilly & purposely to humiliate you in front of others...my guess is the latter.

Since you are not a very aggressive girl good at exchanging words with others, you suggest you not say anything overboard. However, I agree with others that you _definitely_ need to put a stop on this.

Be calm, _firm_, and straight forward (look into her eyes): Are you serious or just joking? if you are joking, I found it's not funny but rather irritating.

Don't be mad, believe me. For someone I don't like (or who I hate), I would actually be thrilled to hear if he/she said somthing like this in purblic...as it only demonstrates how low he/she is.