I think you should see a doctor
to find out what these bumps are, and how to treat them appropriately.
There are many reasons why a person can suddenly find pimple-like bumps or a rash on their chest hand and arms, and generally these are harmless in nature.
It really depends and given that your question is very generalized, it is hard to predict what your particular trigger was in this case. Most rashes are chronic in nature. Irritation of the skin by allergies or other irritants can be due to eczema, a heat related rash or a manifestation of another chronic disease. Sudden rashes are sometimes also seen due to an allergic contact dermatitis (i.e. poison ivy
or nickel related). You may also want to consider a new soap, item of clothing or another exposure such as a sunburn. If you are having symptoms of fevers or chills or changes in weight, you may want to see your doctor to discuss these symptoms. At that time, your doctor maybe able to ask more relevant questions and come up with a diagnosis.In addition, a very important component of diagnosing a rash is to actually see it. The appearance of the rash, and its distribution can help significantly in making a diagnosis. Depending of your situation, it maybe relatively simple to formulate a treatment strategy as well. Common rashes can be easily treated with topical steroid creams. In either event, I would recommend you make an appointment with your primary care physician
to address your concerns.