Hello dear,
First of all, 
HIV is not diagnosed only on the basis of symptoms.
The presence of 
pimples on chest can be due to either 
hormonal imbalance or due to infection of sebaceous glands or hair follicles.
Kindly 
consult a Dermatologist & get them clinically examined.
Symptomatic relief will be achieved with the application of 
Calamine lotion, intake of anti-histamines & a course of antibiotics.
However, to relieve your concern regarding HIV infection, you can undergo certain tests like p24 assay or PCR for HIV, which becomes positive within a week and are almost 100 % sensitive.
Even the presence of a single virus in the body can be detected by PCR.
So, if this test comes negative, you can be sure that you are safe.
Wishing you a good health.
Take care.