Ms Oosthuizen, who lives near Salisbury in Wiltshire, was competing in the Army's annual tennis competition in 2014 when she hurt her right foot but put it down to a sprain and assumed it would get better in a few weeks.
After six months of constant pain and undergoing various scans and X-rays she was diagnosed with Complex Regional Pain Syndrome (CRPS), a condition thought to be caused by nerves telling the brain a part of the body hurts, even if it is no longer injured.
Eventually, the pain meant she had to leave the Army and, despite concerns from some doctors, she decided to have her lower leg amputated.
"It was becoming intolerable to live with and because of my relative young age and relative good health I thought I can't live like this," Ms Oosthuizen said.
"This was very much a last ditch effort to see if this would make it better and if it didn't work I would just have to accept living with the condition.
"After three-and-a-half years I sat down and gave it proper thought and speaking to medical professionals and some other people that had amputations with this condition I decided to go ahead."