I feel there is a very small stone when I touch my testicles

Your GP should be able to arrange an ultrasound.
I saw my GP several years ago & I said "I found a lump". "Oh, where?"
I glanced down & said "Well, I had 2 - now I've got 3".

Ultrasound showed "multiple serpiginous" (kinda like varicose veins).
Harmless but annoying.

You won't know until you get it checked out.
It's natural for your mind to go to cancer for any strange lumps, but an ultrasound or MRI should give you peace of mind.
Good luck.

If your GP doesn’t do this, get ahold of a urologist, they specialize in this type of thing

I also have "3" testicles. LOL. My urologist did his "light" test, found that it is a spermatocele. It started like a small "stone" at first, and now it's about 1/2 the size of a gonad. It hurts rarely. Ask your GP or urologist about it.

From Google: A fluid-filled cyst in the testicles is called a spermatocele, also known as a spermatic cyst or epididymal cyst:
What it is
A spermatocele is a noncancerous sac that develops in the epididymis, the coiled tube that stores and transports sperm.

Symptoms
Spermatoceles are usually painless and feel like a smooth, firm lump in the scrotum. They may not be noticeable right away.

Cause
The exact cause is unknown, but it may be due to a blockage in the sperm transport tubes. Infection or inflammation may also increase the likelihood of developing a spermatocele.

Treatment
Most spermatoceles don't require treatment and don't reduce fertility. If a spermatocele becomes uncomfortable, a doctor may suggest surgery or recommend over-the-counter pain medication.
Prevalence

Spermatoceles are fairly common, affecting nearly 1 in 3 adult men.