Sore throats are typically caused by one of several common viral infections, like the common cold. As such, it is very common to contract one of these infections multiple times per year (think of how often the average person 'gets a cold' for example). Unfortunately, removing the tonsils does not prevent contracting common viral infections, as the viruses can easily infect the cells in the lining of the throat just as well as they infected the tonsils.
The idea behind taking out the tonsils is that this can help decrease the severity of the sore throat
episodes. Tonsils are removed typically only in cases where an individual has had multiple episodes of very severe sore throat where the symptoms are aggravated by swelling and pain in the tonsils. Removing the tonsils will prevent this swelling and the component of pain that was coming from the tonsils, but it does not mean that you still will not have occasional mild sore throats.
I recommend discussing this issue with your primary care doctor
or the ENT doctor who performed the surgery
. If you feel that your sore throat symptoms are severe, your primary care doctor would be happy to examine you to make sure that you are not dealing with a serious throat infection.