Hey, I've also had this type of problem with fastbuds420 seeds. they are more sensitive to over watering or have more genetic variability.
when my plants did this all the other plants where fine, two of the fast buds looked like this. I watered them all the same.
so its hard to tell if it was over watering if other plants where a-ok, or was it that this strain is extra sensitive to wet conditions or other aspects in the environment.
Let it dry out some and do not give it any feed, it needs to recover. the medium its in should have enough nutes to last it till 3 or 4 weeks.
Mine bounced back and turned into descent plants but it will set you back by a week or so.
link to my old question.
growdiaries.com/grow-questions/11486-bad-seed-or-something-else
best of luck