Discontinue use until you speak with a doctor or pharmasist.
Are you taking the correct dosage for your age, or are you abusing it and taking much more, is the question?
Uncontrollable shaking can be a side effect of Sudafed. You can call the pharmacist at the store where you got the medicine-- or any store pharmacist, really. If you saw a doctor, you can also call them to ask.
If I were you, I'd definitely call the pharmacist to make sure it's not a serious side effect. It may be that you accidentally took a dose that was too high, or took it too soon after the last one, and a pharmacist would probably be able to advise you on whether it's safe to take again and how much can safely be taken. The pharmacist may even suggest a different medicine that he/she thinks will be more appropriate for you.