Whenever I put weight on my hand, it hurts a lot like if it was broken. It's been 3 weeks already. It doesn't seem to go away. But it's only when I put my weight on it.

Put weight meaning for example I'm sitting down and I push against the chair to get up.

Can this be arthritist or something?
I'm in my early 30's and I have skinny hands ( I've been told by doctors I may have some form of marphan's disease)

Did you injure it 3 weeks ago?

no. not at all