I would start to worry when you start seeing a pronounced triangle shape to her body, but I would pick up a digital scale to be sure ($15-20 at Walmart).

She may be not eating since your temps are on the cooler side, I'd bump her hot side to ~90 and her cold side/ambient temps to ~80/82 and see how that works. If you could describe your setup, how you're measuring temps, and how you've been offering food (live, f/t, etc) we may be able to offer suggestions to get her eating again.