Gemma Underwood attended her regular class last month [March 28] and woke up feeling sore the following day.
The 41-year-old said she ignored her soreness as she had an 'intense' workout and would often experience DOMS [delayed onset muscle soreness] after classes.
But when Gemma woke up the following day she found she was unable to move her upper body and had severe pain in her shoulders and arms.
The orange-belt athlete said her arms ballooned as the muscles were tight and she couldn't get out of bed.
When the mental health ward manager noticed she was passing very dark urine that 'looked like cola', she was rushed to A&E.
There, blood tests revealed Gemma had rhabdomyolysis, a life-threatening condition causing muscle breakdown.
The condition is often caused by high-intensity exercise and causes muscles to break down and release harmful substances that can cause organ failure and death.
Gemma, from Ayr, said: "I attended my regular karate class and on Friday I felt a bit sore but quite normal for the day after strenuous exercise.
"I first noticed something was off when I couldn't move my upper body to get out of bed on Saturday morning.
"I thought I had just over done it a bit at karate but the pain was disproportionate to the workout.
"I always get slightly sore muscles after exercise but certainly not as intense.
"I was trying to sit up to get out of bed when I felt the muscle pain but was unable to. The muscles in my arms became very swollen and tight.
"When I noticed my urine was very dark I looked online to see if this was a symptom of any condition, it quickly alerted me to rhabdomyolysis.
"It's something that can happen to anyone and what doctors said was that I wasn't hydrated enough while doing an intense workout. I should have been drinking more water before.
"It felt really scary, it can be life-threatening so I panicked.
"I was very scared knowing I could have died but that gave me the motivation to go to A&E because obviously I didn't want to die.
Gemma stayed in hospital for five days where she was put on IV fluid and had a catheter fitted.
Now home, Gemma is resting and drinking plenty of fluids as recovery can take months depending on complications.
Despite being out of danger, Gemma said her muscles are still sore and the experience has left her feeling drained.
The gym-goer is now warning others to stay hydrated before working out and to watch out for symptoms of rhabdomyolysis.
Gemma said: "I'm very drained, my muscles are still quite sore.
"They said treatment can take a few weeks or a few months depending on complications.
"I am currently resting, no exercise at all for at least three weeks, and drinking plenty of fluids.
"It's really important to know the symptoms, which are muscle pain, weak muscles and dark urine.
"Make sure you're hydrated before any workout, it doesn't matter how intense it is."
WHAT IS RHABDOMYOLYSIS? (Info from NHS)
Rhabdomyolysis is a potentially life-threatening condition caused by the rapid breakdown of damaged muscle with the release of the intracellular muscle contents, including myoglobin, creatine kinase and various electrolytes, into the bloodstream and extracellular space.