Is it normal my finger hurt extremely bad when i flex them?

I've very, very, very seriously played guitar and piano for nearly 25 years, used to give lessons, and am into competitive Rubik's Cube solving which is also very reliant on dexterity. I didn't originally have any problems. My fingers were always crazy fast and strong. This is because they progressed gradually.

I'm ambidextrous so a few years ago I decided to start learning to play guitar left-handed out of curiosity regarding if I might have been better that way. All of a sudden, boom, problems like you're describing.

I came to the conclusion that it was the result of my fingering hand (now the right one) being asked to do things it had no experience doing by the brain of a guitar teacher who knew pieces composed by virtuosos, asking it to do things a normal beginner wouldn't even know how to ask of it. In other words, I was abusing the fuck out of it.

Is it possible you've been grinding really hard? You have to remember there's a component of playing that's essentially bodybuilding and gymnastics for your hands. It's tempting to practice 8 hours per day but depending on what you're doing, you have to factor in recovery. Hands aren't really made to do the sort of things instruments require and they can only slowly be changed without serious issues.

Look up hand and wrist exercises on the internet.

It is likely that the pain you are experiencing can be resolved though stretches and exercises. You don't have to buy anything for most of them; and if you do need to buy something - its not expensive.

Then don't do that.

You might be giving yourself arthritis from overuse or something- I don't know, I'm not a doctor. You know who would know, and is a doctor? Your doctor. Go talk to them.

Definitely something to see a doctor about