Well I probably can't ease your mind with my story, however it will make yo ufeel better if it turns out to be a break.
Didn't they take x-rays in the ER?
When my oldest was 22 months old, he tripped and fell while walking down a carpeted ramp at hte childrens museum (in the toddler play area of all places). He wasn't running, or skipping, or being goofy. He was walking down the ramp holding onto the railing, tripped and fell onto his hands and knees like he has done a million times before. He cried and cried and wouldn't stand on it and I just assumed it was a sprained ankle too. The museum filled out an accident report in case we wanted or needed to go to the hospital later. We get home, he still won't stand on it, and took him into the ER and after x-rays learned it was broken, spiral fracture. He had a splint for 2 days, then had a cast set on it for 4 weeks. It really sucked at first, but really it wasn't that bad and it went by so fast.
And now at 4 1/2 you'd never know he'd had a broken leg. Its completely healed and perfect. So even if it turns out to be broken, don't be too upset, it goes by fast, and your baby won't remember it, and it probably won't bother her if its in a cast. My son walked on his after a fe wdays, and was running on it after the first week. Crazy.
I've used an exersaucer with both my boys with no problems. My oldest walked at 9 months, my youngest at 11 months. I think they are fine if you use them sparingly, like to get something done or shower. I'm sure it could have been the cause, but really I'm sure it was just the way she jumped or moved and it could have happened while she was standing on your lap, up against the sofa, learning to walk, anything. Don't stress about that.