Getting warmer

I'm in my 4th year of studying, adult learner. I'm noticing that lately (last year or so) I get warmer when playing long-ish pieces.

I'm not physically tired from playing, and my hands are relaxed, and I can keep going, I just feel noticeably warmer, like if I had to have a t-shirt and a shirt on when I sit down to practice, after 20-30 minutes of it I not only have to remove the shirt but also get my little fan working to cool down a bit. It is more noticeable when I'm playing somewhat longer pieces from start to finish. "Longer" for me is like 6 pages.

Do you experience the same? Is it normal?