The 27-year-old "Bang Bang" singer felt she should be honest with her fans after receiving numerous messages of support, but said she was struggling to stay positive, reported Daily Mirror.
"I have seen a lot of "How are you today" comments... How am I.... *Looks up sighs and has to be honest," she wrote on Instagram.
"I am not ok... I am in a lot of pain. But I'm working on it with the doctors. We all have our moments and one of mine is now.
She thanked fans for their support, adding it is helping "so much" and hoping that by being honest she may help herself recover.
The update comes after the "Domino" songstress had to cancel concerts after taking longer than expected to recover.
She suffers from a heart condition but insisted earlier this week her latest trip to hospital was nothing to do with her voice or heart, although she revealed no more details.