Apparently her temperature was tested seven times whilst she was at Heathrow because she told officials that she felt as if she might be developing a fever. However, each time her temperature was below the UK algorithm threshold of 37.5 degrees centigrade (99.5 degrees F) - hence she was allowed onto the flight to Glasgow.
Anyway - she has progressed to the next stage and today is described as 'deteriorating and in critical condition'. I expected her to deteriorate for a time at least, but the use of the word critical and the tone of the announcement do seem to paint a picture which suggests she may not be responding as well as (say) the two nurses from Dallas.
I do hope that even if the anti-viral drug she received is not proving effective, the serum anti-bodies and robust supportive care will help her to beat this.
http://www.bbc.co.uk/news/uk-30666265
Sounds like she felt it coming on. In several people who got Ebola in US, fatigue was the first symptom, not fever.