It is possible that you also saw your cockatiel shaking right after you gave it a bath, just as some people have claimed they did.
01.
If its wings are a bit droopy, if it does not respond to you, if its feathers are ruffled, or it is exhibiting other unusual behaviors, then it is most likely sick.
02.
This will be especially difficult to notice in your cockatiel. This is because cockatiels like to keep such behaviors hidden.
03.
Shaking is generally normal bird behavior. If you see your cockatiel shaking, it may just be doing so to relax.
04.