The red spots inside are most likely chapped spots. Summer people are prone to heatstroke, but we won't notice it when it's not serious. In yoga practice, some arms will be folded and squeezed into the inner side of our arms. The spots come out like this.
It doesn't matter, it will disappear in two days ~ don't be afraid of yoga because of this, and reply to your yoga instructor in time to guide you to practice yoga more safely and comfortably ~
I hope this answer will satisfy you.