A festival-goer said she woke up with a severely swollen face and was forced to go to hospital after she suffered from "sun poisoning".
Ruby Brewer, 23, said an adverse reaction to the sun caused her face and neck to erupt in a red rash.
Her eyes also swelled so much she was forced to see through small slits.
Ruby attended the Love Supreme Festival in East Sussex with her friends on July 2, but forgot to bring any sunscreen.
On July 4 she woke up at 8am to a shocking image in the mirror.
Her face had swollen to twice its usual size and she had a rash on her arms and neck.
Ruby was rushed to hospital and says she now thinks her unusual reaction was a case of ‘sun poisoning’, called polymorphic light eruption by the NHS, which has left her scared of the sun, the Manchester Evening News reports.
She said: "When I woke up, I went to open my eyes and that's when I realised I couldn't open them fully. I was really shocked when I looked in the mirror and saw how big my face was.
"My face had doubled in size and there was just a small slit that I could see [through]. My skin was all blotchy and red and my arms and neck had rashes on them.
"I looked like Sloth from The Goonies or even a chipmunk as my cheeks were so large.
"My mum jumped out of bed when she saw me and said 'don't panic, we're going to go to hospital now'."
After a 10-minute drive to Princess Royal University Hospital in Locksbottom, London, doctors at the A&E department asked Ruby if she was aware of any allergies she had.
She was hooked up to an IV drip that provided her body with antihistamines and steroids to help reduce the swelling.
Ruby said: "My mum guided me in and spoke to the receptionist. As we didn't know what it was, she said that I'd had an allergic reaction.
"I was asked if I had any known allergies, which I don't. They then put a cannula in, gave me medication and monitored me to see how I'd react.
"They gave me an antihistamine injection and steroid injection that did slightly bring down the swelling. They monitored me for five hours and the swelling did go down, they then prescribed me steroids and antihistamine and sent me home."
It took three days for Ruby's face to begin to return to normal and it was initially believed she suffered an allergic reaction.
However, after researching her symptoms online, she now believes she suffered sun poisoning - a severe reaction to sunburn.
Ruby added: "When I got home I said to my mum 'it can't be something that I've eaten because I didn't have an internal reaction'. Obviously, I had my festival so I knew I'd been in the sun for a long time.
"My mum asked if I'd put any sun cream on, which I hadn't. I started googling 'sun allergy' and found an article where someone had sun poisoning, I clicked on it and saw a picture of a woman whose face had swollen up.
"I put two and two together after seeing pictures of people who looked like me. It was a bit of a relief to know what it was.
"Now I'm just scared of the sun. I slowly deflated over those few days thanks to the steroids and I was back to normal by Thursday.
"My skin was super tight, super itchy and if you touched it, it was really scaly - it felt really horrible."
To help avoid a similar incident ever happening again, Ruby now covers herself with SPF 50 daily.
She wants to raise public awareness of the dangers of the sun and what people can do to avoid any complications.
She said: "I've got this really good SPF 50 cream that I now put on underneath my makeup and I have the matching spray, which I carry around with me at all times.
"I spray that over my makeup and when it's sunny and I've been out for a long time and I haven't been able to re-apply, I just spray it on top of my make-up.
"I feel more comfortable now that I have that. I would say to anyone who thinks it won't happen to them 'you'll definitely eat your words' because I never thought it would happen to me."
According to the NHS, a polymorphic light eruption can trigger an itchy or burning rash to appear up to three days after exposure to the sun.
The face is not always affected and symptoms can also appear on the head, neck, chest and arms.
While there is no cure for the condition, the NHS advises sufferers to use sunscreen with SPF50 or above and carefully avoid the sun, particularly between 11am and 3pm when the sun's rays are at their strongest.
