Decreased muscle mass

Yesterday, I've measured my body composition at the sports club and was shocked to know that muscle mass of my body has decreased by 0.3 kg in the past one month.  I did machine training for one month and is this the result?  Was my training meaningless? 

However, today, I have muscle pain, that means my muscle is recovering from the damage caused by training.  This may explain the reduction of muscle mass.  Probably, a correct result cannot be obtained before recovery of muscle.  I'll remind it when I measure my body composition next time.