Did you have counseling before you had your tubal ligation? Did they explain the benefits and risks to you? Legally, they are required to do so in order to obtain your informed consent. But if you were delivering your son by C-section, I can see that you had other things on your mind and might not remember everything.
Tubal ligation (whether by tying or cutting and usually with cauterization) is designed to be a permanent and irreversible method of birth control. No one should undergo it if there's a chance of wanting another child. There are some reversal procedures but they are expensive, far from perfect (due to the scarring from the original operation, which is part of why the operation works), and they are considered entirely elective and therefore not covered by most insurance.
There is a very small percentage of failure rates and it's possible, though extremely rare, to become pregnant after a tubal ligation. If a pregnancy does occur, it's statistically more likely than normal to occur in the Fallopian tube. These ectopic pregnancies are not viable and they are extremely dangerous to the woman.
So, once a woman has a tubal ligation, it is presumed that she cannot become pregnant, and no other contraceptive method is needed. However, if she has any symptoms of pregnancy at all (missed period, typical symptoms) or (important!) if she has shoulder pain, it's absolutely essential that she be checked out right away. The shoulder pain is a red flag because an ectopic/tubal pregnancy can press on the phrenic nerve, which refers pain from the abdomen to the shoulder. This is an emergency situation.
So, if you are hoping to get pregnant, you should consult a qualified ob/gyn but not get your hopes up. If you have any pregnancy symptoms, please be seen immediately.