Due to overstrain activity.

Do the cat/cow stretch. Get on all fours, with your arms straight and your hands directly under your shoulders; your knees hip-width apart.

Make sure that you are taking adequate rest on the days that you’re not working out. Include functional training in your daily exercise regime. Also try swimming for a good cardiovascular training. It is very important to maintain a good posture as adapting a poor posture will make you feel more tired since the breathing is disrupted ;and a good warm up and cool down before and after your training session. Regular stretches will help you with the stiffness.