It depends on how long you lifted weights for and if you are getting enough protein which is 1.2 - 1.7 grams per kilogram of body weight per day. Do not exceed 40 grams of protein in one sitting. Make sure you drink about 50 milliliters per kilogram of body weight. Yes, sex does actually increase your testosterone by a little so no it doesn't hinder muscle growth. Make sure you sleep enough at least 7 hours. The last reason I can think of is that you need to increase the weight or your rep count. You also might not be doing your exercises to the fullest so make sure you are doing full reps and not using momentum. Full range of motion.