Am I lifting my weights incorrectly or do I just build muscle quickly?

So I've only been lifting for about a month and only 2 times a week and the past few times, I didn't feel any strain but I think I was lifting correctly. I had my feet at shoulder width apart, I wasn't slouching or moving any part of my body other than my arm, I was lifting at a moderate pace. Then I thought maybe I was getting stronger but it shouldn't happen this quickly...what can I do to see if I can move up or if I was just making a mistake?