break that rotted bud open and search for caterpillars or bud worms. hopefully you dont have them but caterpillar damage looks just like that, they eat and shit in your buds causing spots to die off. im praying for you cus caterpillars fucking suck, i had them in my outdoor grow last year, lost a bit of weed to them and trimming the buds was a pain cus you never know when one would pop out at you. i hate those things
if you find a caterpillar there will be more on other parts of the plant or on your other plants.