Middle finger hurts when I stretch it and completely close it! Really bad pain

So I was walking up the stairs and then my middle finger randomly hurt when I went it upwards (like stretching my finger)… I'm so worried, I managed to find where about the pain is, it's like below the knuckle but it hurt the rest of the bottom of my finger (it's like someone put a needle into my finger there)

So if I stretch it on its own it hurts and then if I get my other middle finger and put that on the bottom of the middle finger tat hurts tip and get my thumb from my other hand and put it over the bottom of the knuckle and I force it to stretch it hurts so much (if that makes sense)
It's deffo around my knuckle area it hurts so bad

I DON't WANT ARTHRITIS OR ANYTHING OMG! I'm ONLY 16!

I use my iphone a lot but all that finger does it hold the phone with my index finger, it's my thumbs that do all the typing etc

I don't know what's wrong with it, it's hurting as I type (on the computer) too (at times, but a mild pain not major)

I don't want anything wrong with it I'm so scared! Is it serious! My mum says it should go in a few days I don't know what's wrong with it!

(Prior to the pain I was stroking my cat in her neck area - they love that LOL- but that didn't affect my other fingers so I dunnoooo)

Keep working your finger