Swollen Red Butt - Had to Cull - Then disected

My oldest RIR hen has had something hard growing in her bottom for some months now. At first a few months ago I thought she was egg bound and I did the epsom salts bath with no relief. Her bottom stayed messy with poo all the time. Yesterday and this afternoon I noticed her just standing and not looking good, so I did the epsom salts bath again with no luck. While bathing her I noticed how thin she was and the little noise she was making trying to breathe. She had been walking with her legs apart for a while now but today she was barely moving. I know I may be rambling and I am sorry but we made the decision to cull her tonight. Since the poor girl seemed to be hurting, I feel it was our only choice. After my husband culled her, I decided to do some investigating and did an autopsy on her and I found a huge tumor. Now I know what the problem was and I know she must have been soo miserable. I loved her so much but now I'll know if this happens again what it is....