Hello, I'm a 19 year old female and I found a hard painless, immovable, swollen lump under my chin, right in the centre, whilst on holiday in 2020. I put it down to a gland that was swollen from an infection and tried to ignore it. 
The lump did get smaller and smaller, but it's never fully disappeared. Since then, it has fluctuated in size, it could be barely noticeable for months.. and then for a few days it's really big again.
I went to the doctors last year, he felt my throat and just said that I'm "too healthy and young to have cancer". I took his word for it and pushed it to the back of my mind as I HATE the doctors.

But last Tuesday I woke up with a sore throat and the lump was so big and swollen again. But this time it hurt and it was that big I could feel the pressure of it against my skin. It's now a week later and the lump has shrunk back to its original size again and there's no pain at all.
Do cancerous glands fluctuate in size? I have no other symptoms.