Without being able to see it, it sounds like an infection. You should go to the doctor and see if you need an antibiotic. My youngest sister had this happen to her when she had the chicken pox. One got infected and a red line started to appear then grew up her arm. She actually had to be hospitalized for IV antibiotics. But this was a long time ago. Just have it looked at to be safe and treat it sooner than later. :)
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