Best guess... She grew up with a stutter, achieved fluency, but the slips back into her stutter around you. Your stutter may remind her of her own. I work with people for long stretches (months) before anyone hears me block significantly enough for it to attract attention. I sometimes receive incredulity from them when I tell them I am a stutterer. You might talk to her about your stutter. Always nice to meet one of our own in the wild.