The 22-year-old singer was criticised when she gained weight last year and was forced to defend herself.
Now, Wiseman has revealed the real reason behind her weight gain, and says that her current weight is her natural size.
She told British New! magazine: 'I had an operation at the start of last year. I had a cyst on my ovary and it made me put on a lot of weight.
'I went a full dress size bigger - from a size 8 up to a 10, maybe even a 12. Everyone thinks I've tried to lose weight, but I genuinely haven't.'