Aah common la tu after vaccination, arm soreness is super normal, especially on dose 1! A few of my mum friends had the same thing, some lasted a day or two je. Try applying warm compress, or massage the area gently kalau boleh; paracetamol also helps if dia uncomfortable sangat. Tapi if the numbness or soreness doesn't improve in a few days or gets worse, better check with the doc just to be safe, okay? Your little one will be fine, don't worry!