What the hell is going on with my arms???

So, I'm pretty new to working out. I'm deceptively strong, but I'm more into losing weight than strength building. Still though, I paid $600 for a year of gym membership, and I bought these things called Powerblock's, which are like dumbells, but you can customize their weight up to 50lbs (and you can expand them later up to 90lbs). So I figure I should just do some strength training anyways. Not like there's a downside, right?

So for the first month of this, I had a very wrong idea. I thought "go to the gym, every day, and just do whatever".

So most days were arm day. I'd go on a Monday, arm day. I'd go on a Tuesday, arm day. I'd go 4-5 days a week.

Since then, I've been told NOT to do that, and only do arm day once per week. As the muscles need time to heal between lifts.

Here's the thing I don't get. I've been doing curls, and another machine where you pull these pulleys downward, and set the weight. With Curls, I'm up to 30lbs for that. Sets of 12 is what I was told. I was doing sets of 20, but I was told that was too much.

When I got home, I tried doing this other exercise with my Powerblock. I'm sure there's a name for it, but bear with me. You stand straight up, with your arms at your sides, holding the dumbells. Then the exercise is, you raise your arms. Not forward, not towards you, but to your sides. So your body makes a T shape. Arms fully extended left and right at shoulder height, still holding the dumbells. and then you lower your arms back to your waist. It's kind of like flapping your wings, but in slow motion, and more stiff.

I can't do that at 30lbs. I can't even do it fully at 20lbs. I've been doing it at 17.5 lbs. But here's what's weird. Even from day 1 when I did curls, I could either do it, or I couldn't. Based on weight. My first attempt I tried a 50lbs dumbell. Mistake. On day 1 I kept sizing down until I got to the 10lbs, and then over the coarse of a week, moved up to a 20lbs, and now a 30lbs. But if I tried to do another weight that I couldn't lift, I just couldn't lift it. There's no pain.

But when I do my T arms exercise, even at 17.5lbs, there's pain. It's not a lot of pain, but there's pain.

There's no pain if I go down to 15lbs, which is the powerblock with no weight, and completely empty. So it's practically weightless. Which is what I don't understand. Why can I lift curls at 30lbs, but this T exercise not only causes pain, but I also can't even do 20lbs?

Am I doing something dangerous?