Today was my upper body day and I was doing bicep curls 8/8/10 at 30 lbs. I'm on my third set and try to do my 7th rep. I get the dumbell half way up and it sticks so I put everything into lifting it up and my arm goes down. It's like my arm just gave up on me. What is this/why does it happen? Keep in mind I'm new to weight lifting altogether.