Three weeks later, in the middle of a Year 10 practical, her legs "got a bit shaky". Within the hour she was in hospital, paralysed from the waist down.
Having never taken a day off ill in 17 years of teaching, she found herself on long-term sick leave, and spent six months in two hospitals undergoing rehabilitation and physiotherapy. Doctors believe a virus picked up on holiday caused an inflammation of her spinal cord, leaving her in a wheelchair.
But just before Easter she started back at work as head of department at Radyr comprehensive in Cardiff, and is looking forward to teaching three days a week after the holidays.
Headteacher Steven Fowler encouraged her to return, and the school and governors worked hard to make the physical adaptations needed to accommodate her wheelchair on a hillside site. These included ramps, a carport, automatically-opening doors, and a specially-adapted lavatory. She also has a Tablet personal computer, and an assistant for practical lessons.
Miss Lloyd, 39, says: "The headteacher was superb. He came into the hospital very early on and said, 'Do you want to come back?' I said it was unlikely I would walk again. He said, 'So you can teach sitting down, then'.
"My union's line was 'You are head of home economics at Radyr comprehensive, and that's the job you are going back to'. I decided if they thought I could do it, then I could."
Things did not happen as quickly as she would have liked. She was pronounced fit last August, but it took until nearly the end of last term before things were ready for her. Nonetheless, Gethin Lewis, secretary of the National Union of Teachers Cymru, said Miss Lloyd's case was a model of good practice. Many teachers with lengthy illnesses do not receive such positive treatment, he warns.
In some authorities, sick teachers face "back to work" interviews with line managers after only three days off.
"These rigid absence monitoring processes are not seen as supportive in any way," he said.
"I was away from school for almost 12 months, having treatment for cancer.
I wanted to return, but if I'd had an interview every time I was returning from chemotherapy, it would have been more stress for me."
Meanwhile, Miss Lloyd says it was the support of colleagues, parents and pupils, and the thought of returning to the job she loves, that kept her going.
"I was desperate to get back, to return to normality. The only thing that kept me going while I was in rehab was that I would still be able to have my life."