Chart star Jessie J has pulled out of two weekend concerts after being diagnosed with a "fatigue" virus.
The singer apologised to fans for cancelling her show in Pontypridd on Friday, the first part of Ponty's Big Weekend.
She also pulled out of a show at Alnwick Castle in Northumberland on Saturday, which has been rescheduled for August 25.
Show bosses said the singer was suffering from cytomegalovirus (CMV) and doctors had advised her to take a rest.
In a message on her website Jessie J said: "I have been diagnosed with a viral infection recently that has caused me to be extremely fatigued.
"I was hoping to be better, but I have been to see my doctor this morning after not feeling great who has advised me I am still in need of rest and to perform will make me worse."
Jessie J, a judge on BBC1 talent series The Voice, said she wanted to personally apologise to fans and was "just not well enough" to appear.
"Back to bed I go - so sorry," she added.
The Price Tag star had to cancel a number of shows last summer after damaging her foot in a fall, but she did appear on stage, notably at Glastonbury Festival. She performed sitting on a throne with her leg in a plaster cast.
CMV is said to be a common virus with no cure. It is part of the herpes family.
Christian Hanagan, of Rhondda Cynon Taf Council, said: "We wish Jessie well in her treatment and hope she is back on her feet soon."
Fans have been told they will get their money back.
Further Ponty's Big Weekend shows starring Olly Murs and Alfie Boe over the weekend were unaffected.