What could cause an inflamed and painful sac of yellow or white stuff under my left tonsil?

Hi,

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The tonsils are red and swollen, and the yellowish thing could be either follicle studded with pus or a stone formation. The symptoms point to an infection. Amoxicillin of appropriate dosage for your weight and age should be taken and the course is for five days. If it does not get better, get a culture swab done and check what antibiotic it is sensitive to, and change accordingly. You may also use Benzydamine gargles diluted. Take Tylenol (Acetaminophen 500 mg) for pain and systemic decongestant if there is a lot of mucus and voice change. Avoid spicy, citrus and chilled items. Of course, avoid smoking and alcohol. For any further information, please revert to me. Glad to help you.