The 39-year-old was taken to hospital in Gloucester on Sunday and suffered the effects of dehydration.
She was discharged later the same day and has spent a few days recovering.
But she was back in the water yesterday to continue her bid to become the first woman to swim the length of the River Severn, the longest river in England and Wales.
When she was hospitalised, she had just 20 miles to go of the 220-mile challenge.
Melissa, a nurse in the intensive care unit at Royal Shrewsbury Hospital, is now hoping to reach the finish line at Severn Beach in Gloucestershire tomorrow.
Speaking yesterday before going back in the water, she said: "I'm getting better, I'm keeping food and water down.
"I'm hoping to get done on Saturday and be back in work on Monday."
This final section of her swim is the most dangerous as it has the second highest tidal range in the world.
So far Melissa has battled swollen waters, rain and less than summery conditions in her bid to travel from the source of the Severn at a Mid Wales mountain to the Bristol Channel.
Melissa, who lives near Shrewsbury, has now raised more than £2,300 for the charity Versus Arthritis, as a thank you for funding her stem cell treatment.
She needed stem cell therapy for osteoarthritis.
Melissa said: "It's lovely people are so generous. They are so supportive."
Last year she swam 22 miles across the English Channel as part of a relay team of four for a charity effort.
To sponsor Melissa’s swim, visit www.justgiving.com/fundraising/swimmingmelissa-compton
Follow her progress on Facebook, searching for Swimming Melissa Compton