A woman who donated her kidney to a stranger said she was "disgusted" by her treatment in hospital and that "it has left a bad taste" in her mouth.
Jacqui Collins, from Clayton-le-Moors, Lancashire said she was made to feel like a "nuisance" by Manchester Royal Infirmary staff after surgery last May.
The former regimental sergeant major, 54, said errors in her care included problems with post-surgery pain relief.
Central Manchester University Hospitals NHS Foundation Trust has apologised.
Ms Collins told the BBC she decided to become a "living altruistic donor" - someone willing to give away an organ to potentially save the life of a stranger - because a couple of Army friends had donated kidneys to siblings.
She said a cannula used to administer pain relief had not been inserted properly, causing excruciating pain when the anaesthetic wore off hours after the procedure on 25 May 2017.
The project worker for Veterans in Communities told the Lancashire Telegraph she informed three nurses that her drip had not been attached properly, and that she was also refused painkillers.
Ms Collins also said her hospital gown had twice not been put on properly, resulting in it coming open and potentially exposing her to other patients.
And she said she had been "dragged, while screaming in pain" out of bed less than a day after the operation.
Once she had been discharged, she said she was stopped from getting into a taxi home because she had wrongly been given another patient's medication, including morphine.
Ms Collins also said she had to argue for months to get her travel expenses of about £400 - covering 20 appointments - fully refunded.
Ms Collins, who has raised £35,000 for charity with her mother Mo and best friend Debs Keenan since leaving the Army in 2014, said: "I should feel pleased for saving someone's life but I feel let down.
"You wouldn't believe the nightmare it turned into.
"I absolutely wouldn't advise anyone to choose to do it for a stranger after the way I was treated... and I am still in pain today when I lift things."
The hospital trust said it has conducted a full investigation and "made changes to prevent it happening again".
- A living person who donates one of their kidneys to someone they do not already know is called a non-directed altruistic kidney donor
- Hundreds of people have now donated anonymously their kidney to someone on the national transplant list
- A volunteer donor will have to undergo a number of medical and surgical tests to check they are fit enough to donate
- The risk of death for the donor is estimated as one in 3,000, similar to having an appendix removed
Information from NHS Blood and Transplant