She probably just bruised it, that area can be painful I'm sure if you bruise it since you sit on it every day etc. I would think if she fractured something she would be in more pain but then again I don't know, never had that happen. My 13 month old bathes in the bath with her older sister and has for months, she's too big at this age for her bath seat anyway so you are not a bad mom
if she continues to show pain and you're concerned I would take her to the doc and have it checked out. Good luck, hope everything is ok.