Fitness drop after 1 month rest

Since back from New Zealand, I suffered from muscle pain in the Gluteal and Icheal Tuberosity, due to sitting for 10 hours on a long-haul flight.
Then I catch the flu and cough, so I rest for nearly a month.
After short riding on a home trainer for 3 days, I felt my Icheal tuberosity could withstand the hard Fi'zi:k seat.

Still not fully recovered from the cough, today I bravely ride under the hot evening sun.

The above screen captured from Endomondo shows my fitness on 25 Feb 2017
and today 25 March 2017. Today heart rate 40%  is at the maximum Zone 5.