I'm very sorry. :hugs If you decide that is the best thing, and it often is, I would recommend necropsy, so you know the cause. The knowledge can help in the future, and can bring peace of mind in many cases.
Does her abdomen feel bloated, below the vent between the legs? Her stance suggests reproductive problem. It's not uncommon, particularly in birds over the age of two, and in breeds that were bred to lay lots of eggs, which she is. It could be an infection like EYP or salpingitis, or a...
If getting fecal test isn't doable (many vets won't do it without seeing the bird. If you have a vet for other animals it cost nothing to ask) then I would consider worming her to see if it helps. I would use Safeguard liquid goat wormer or Valbazen (labeled for cattle). The Safeguard is...
Has her behavior changed?
As said before, calcium citrate is available at any pharmacy or big box store like walmart.
If she is egg bound, then time is of the essence, it can be fatal if not resolved in fairly short time.
Is she pooping? You said there was a mass below the vent, is it hard...
Many calcium citrate +D tablets are 650 mg, just give one daily. The petites are usually 400 mg, I usually give two of those. The calcium will help with contractions and help her pass an egg, if that's what's going on. You would have to give large amounts for a long period of time to cause a...