I agree with appylover. The BO probably just felt like she needed to blame somebody.
A dirty stall and less-than-perfectly-clean water is not going to cause a horse to colic. My guys slobber grain and half-chewed hay into their water buckets all the time and they're fine. It's not drinking enough
water that can make a horse colic.
It wasn't your fault. The BO sounds like she's just being a jerk. From what you're saying, it sounds like the BO was being weird about it. As in taking it too lightly or something. I know I wouldn't be laughing or cracking jokes if one of my horses was deathly sick.