I imagine you might be cramping up because you're too tense when you play. It feels like you can't play fast this way at first and might seem awkward, but you want to be loose and relaxed. Maybe something is slightly off with your position and is straining you more than normally, no way I could really tell though I would just say keep practicing and it will go away with time but if it persists and becomes a large issue maybe get professional help to be safe.
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