A single soft mobile swelling under the chin at 26 is almost always a reactive lymph node - from a throat infection, a dental problem or skin infection nearby. Do not panic about cancer; those nodes are hard, fixed and painless and come with weight loss or night sweats. Reactive nodes shrink slowly and can take 3-6 weeks even after the infection clears, so 5 days of Clavam not "melting" it is expected and stopping antibiotics on your own is worse. Get your teeth and throat checked - a bad molar is a very common cause.
Next Steps
Complete the antibiotic course as advised, get a dental check, and get an ultrasound of the neck plus a CBC and ESR. If it grows fast, becomes hard and stuck to the skin, or you get fever with weight loss, it needs an FNAC. Book a consult with me on Practo with the scan report and I will tell you clearly whether it needs anything further - most of these need only follow-up.