I should start out by saying that I'm not a doctor. I'm assuming that you're an adult. If you're under 18 you should speak to your parents.
Please be reassured this is highly unlikely to be cancer.
You may have a throat infection or trauma due to over-use, and the lump is probably a lymph node (sometimes called a "gland"). It is a part of the immune system, and its job is fight nearby infections. While doing this, it will get swollen and painful - but it's nothing to worry about.
I suggest you treat the pain with soluble paracetamol - two tablets in water every four hours, plus a throat soother. As an amateur singer, my favourite throat soothers are called Vocalzones and they're available in most chemists. To this, you can also add EITHER aspirin OR ibuprofen but not both. For all these medicines, you MUST follow the dosage instructions on the label - it is dangerous to overdose. Finally, give your throat a rest for a few days. No singing or shouting.
With a little luck, your throat should start to improve within 48 hours, and be back to normal within 7 days. If you're worried, then the next step should be to see the pharmacist in your local chemist, and take whatever advice they give you. You probably don't need to see your GP unless the throat fails to improve within 48 hours despite these measures, or the pain gets significantly worse, or you're still having problem after 10-14 days.
