Hello,
These small bumps can be due to contact dermatitis or allergic reaction to the socks, shoes etc ,
heat rash, insect bites or lichen planus if the
rash is violaceous in color. I suggest you to get it evaluated from a dermatologist to confirm the diagnosis.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your dermatologist.