I would definitely speak to your line manager - up to you whether you want to make it formal or informal. You're line manager said it was ok for you to take a break so its none of your colleague's business what you are doing, and you shouldn't have to justify your actions to her, only to your line manager.
You have a right to keep your medical condition confidential, so you didn't even need to explain to her what you were doing - I do understand why you did - but I am just saying its completely none of her business.