Discover APR Increased

I checked all my Annual Percentage Rates today on all my Credit Cards just for myself and have it known.
I jot down how much I can have for Bank Cash Advances (which is really helpful even if I never tried it before), Fees for those Bank Cash Advances and Annual Percentage Rates. I have little knowledge about Credit Cards. That's why as much as possible, I'd like to share it to as many people as possible. There's no such thing as too much reading. And lately, that is what I've been doing.
So after writing whatever I can for my list, I found out that my APR for Discover has Increased! Not only that for also as the fees for the Bank Cash Advances. 
I have no idea that APR can go up and down! Check the links below for my experience before hand and some articles about APR being increased or how to lower them.

Discover Credit Limit Increased after four months story
Money Under 30
Small Business

Last Statement 2015 APR: 22.99%     Cash Advance Fee Before: 24.99%
First Statement 2016 APR: 23.24%     Cash Advance Fee Now: 25.24%

I was planning to call Discover as soon as possible but wonder if I should research about this first.
So I did researched and it says "Variable" Rate, meaning it can go down and up based on the "Prime Rate". I can't really explained what is the Prime Rate since it's the first time I heard/read about this but still, a little notice would be nice.
Or, maybe they sent out a notice about my APR getting high and I didn't read through it, which is improbable. We check our mails everyday and if I got anything from Discover, and even if it's online, I never saw a NOTICE about an APR, etc. So, I doubt they Discover sent me one about this.

With my other Credit Cards, that I've had way before this one, they never increased my APR or Cash Advance Fee. So with this one not being with me even a year and increased without notice, really disappointing.
Oh, main point of this article, I NEVER missed a payment and always paid in FULL. Discover never charged me an interest because I never let it have a balance. This is why I'm confused why they increased the APR without notifying me or writing an explanation. Anyway, I'm not going to call them asking to lower my APR since I don't really care as of this time, Maybe in the future if I need it but for now, let them have fun with doing this kind of stuff since I will not let it affect me.