Why do i ache so much the day after i do martial arts? is it from lack of stretching after?

Delayed muscle sore: Delayed onset muscle soreness (doms). Depending on what your exercise routine was prior to starting martial arts, if your body is not used to the movements, the activity is new to your muscles. As your muscles become active, the mild strains create microscopic damage to the muscle fibers and causes inflammation. This is normal, and as muscles adapt, it improves. Stick with it; it'll be worth it!

Could be!: Could also be from not stretching enough before class! Could also be from overtraining or if you are new to the martial art or if you are only training once a week. Sometimes it takes more regular training for your muscles to adjust to the exercise, so if you only train once a week your body never really adjusts to the exercise. Try icing the sore areas after you train, it might help the next day