Late to your thread. I'm so sorry that it ended up being a tumor. A swollen abdomen could be different things, but it did look like it could have been fluid etc. It's a hard call when it's an animal/bird and they can't tell us anything. But it's really good that you were vigilant and saw that she had swelling and did something about it. Otherwise your hen may have suffered unnecessarily.
Your hen was a very lucky hen that you cared so much about her.
It's hard to say why she had the tumor. It could be anything from just being an unlucky bird, to it being hereditary to who knows. It's always good practice to keep an eye on the rest of your birds as you're doing...just in case. Chances are that this was most likely an isolated incident.
The very best to you and your flock.