This is because you are causing microscopic tears in your muscle fibers when you strength train or work muscles you’ve never worked before. The pain you’re feeling post-exercise is from these minute tears and is called Delayed Onset Muscle Soreness (DOMS).
This pain is normal and can last between 1-7 days after your workout. As these tears heal, your muscles will become stronger. Over time, your muscles will adapt and the soreness will lessen after your workouts.
To minimize the chance of becoming sore, be sure you warm up, cool down and stretch properly before and after your workouts. Drink enough water throughout the day, and try to stay active on a weekly basis.
To relieve soreness, keep your blood flowing by doing some light cardio or other physical activity, or getting a massage. Even doing some of those unwanted house chores can help relieve some pain.