It's most likely from your arms supporting some of the weight. I went through the same thing when I did Starting Strength. Your back should be supporting everything, with your arms there just to steady the bar. Make sure that you're putting your thumbs over the bar, not around it, and widen your grip as much as you need.
When it happened to me, I skipped a workout to let my arm heal and then deloaded about two weeks worth of progress from all my lifts. I was back on track after the two weeks. I hope that helped.
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