The Scottish singer was the sensation of the Sheffield and Birmingham leg of the tour last week, but decided to miss two appearances in Manchester after consultation with producers.
A spokesman said: "She sends her sincere apologies to her fans for not appearing". A decision has yet to be made about Monday's gig in Glasgow, though she hopes to be back on stage in Edinburgh the following day.
Fans lucky enough to catch the talent show contestant performing were full of praise. She gave a stunning comeback performance on the first night of the tour in the Midlands – just nine days after leaving the Priory following treatment for exhaustion.
The audience got to their feet and cheered as their heroine arrived on stage, elegantly dressed in the same shimmering gown she'd worn for the final, complemented by soft eye makeup and a flattering hairdo.
Any slight nerves soon evaporated with that enthusiastic welcome and she went on to sing two songs back to back, blowing kisses with her arms outstretched to the crowd.
"Everyone was wondering how Susan was going to do," one spectator told the Daily Mail. "But she looked confident and her voice sounded amazing."
Her following means that the singer could charge corporate clients as much as £100,000 per 12-minute set. This is on top of the £8-million fortune she is expected to make from her record deal with Britain's Got Talent boss Simon Cowell.