At the start of the week I did a gym session which I had only done a couple of times this year. I decided last time I felt ok doing the lunge exercise and decided to add more difficulty by adding some weights only 12 Kg (26 lb). However the days after that my right leg felt sore.
So for the following couple of days I took it easy by doing some swimming before gradually increasing my running and then another gym session without the extra weights.
From this I learnt not to increase my gym session drastically in a short time period. I already have used this approach for my other training, so I am not sure why I did not take this advice for gym session.