I'm getting random red bumps on my left arm. pea size. what are the causes of it?

Contact Rash: Typically if you have a one sided specific area it may be related to something you have come in contact with. Make sure you see your primary doctor, and initially look for what may be coming in contact with that area. Also note if there is pus, or redness in the area. Other possibilities include viruses etc, but unlikely just to be in one area.