Knowing where you are located would help as it could be weather related. The size of the container is also needed information as it could easily be rootbound in the pot given those symptoms. But the most common cause of the symptoms you describe is over-watering. Pepper plants prefer to dry out between watering and far too many over-water them causing root rot.

You could have also overdosed it with the Neem or applied it in the direct hot sun. But these are all just guesses without much more info from you.

Dave.