If you think your cockatiel might be SICK, or if he/she is showing ANY signs of sickness, please TAKE YOUR BIRD(S) TO AN AVIAN VET AS SOON AS POSSIBLE!
Please keep in mind that since cockatiels are prey birds, many times they will hide signs of sickness until the very last minute.
If they are already showing signs of sickness, that means that things are already pretty bad. Once a bird starts showing signs of illness, every minute literally counts.
If you have even the slightest feeling that your bird may be sick, please take him/her to an avian vet as soon as possible.
Here is a link to an excellent article from the Birds n' Ways website on signs of illness in your pet bird: