Thank you for asking. Age of 19 is too old to be be of some serious trouble. All this is explained by your recent checking in to Gym activities. Also your weight excess is putting extra burden on your thigh muscles to support.
So its just a muscle strain
/ pull / fatigue
due to lactic acid accumulation due to excessive excercise in to gym. Your body is accomodating to the new transitions. Keep doing the excercises. Use some skeletal muscle relaxants like tizanidine
and some acetaminophen
etc and you will be fine in less than a week. No thing to worry about