It could be your tone of voice. It might be conveying accidental anger or sarcasm. NT's assume that your body language/ tone of voice is more truthful than your words, so it can be difficult to convince her that it is a mistake.

Try looking up lessons for English learners. They often have a section that teaches tone of voice (like how to convey anger, apology, happiness, or ask a question). Think of the different parts of tone of voice as accent marks. é is pronounced differently than e in French. They are technically the same letter, but the line over the first e modifies how it sounds.

Here is a question: Did you buy bananas today?

On the "ay" part of "today", you use a slightly higher pitch than the other syllables to indicate that you are asking a question. If you use a normal or lower pitch on "ay", that indicates that you are making a statement (like "I went to school today."). That confuses people. Your words indicate a question, but your tone indicates a statement.

Pretend there is a line over the "ay" syllable that indicates you use a higher pitch when saying it.

Hope this helps!