Yup, that's definitely bud rot and you need to cut it out of there right quick... use sterilized utensils and spray the cut area with a 2% alcohol solution after you've disposed of what you've cut out.
Now here's the thing - bud rot is not confined to one place and WILL not be confined to just this one bud... it will keep popping up in your grow so as long as you're growing, you're going to have to inspect every single bud every single day CLOSELY... if you see, for example, one sugar leaf that has died and dried up, there's bud rot underneath and inside it... Bud rot spreads by spores and so it's really nasty to try to get rid of - but you've got to get rid of it and DON'T SMOKE IT!!! It will do great harm to your lungs.
If you get a harvest out of this, you're going to have to continue the search for infected buds as you trim and as you dry... I lost my entire summer grow to this nasty fungus! It comes on because conditions are much too humid - and it spreads like wildfire... once it starts, it's impossible, really, to stop it... you can only hope to keep cutting it out and get your grow to harvest salvaging as much as possible...
Once the grow is harvested, completely sterilize your tent/grow room and all utensils, tools, lights, fans - anything that was in the tent - before you start another grow. And for your next grow, you've got to keep the humidity down in the 40s max when in flower.
So sorry you're going through this! It's awful!! Good luck...