Well it can be many factors.
First try to plug it off and see if it goes away easily. If so, it’s recommended to check the inner of the bud and look for any rot. It can be a symptom of bud rot.
It can be your light, probably too close and/or too intense.
Too many nutrients used during end of flowering, you can control the EC level of the soil by taking a sample from the pots drain.
Or just simply the beginning of the senescence, which can be normal at this stage of your growth, particularly if you didn’t use too many nutrients during its life cycle.