Thank you for your query on Healthcare Magic.
If they are exclusively present on exposed parts like arm and legs, they could be due to insect bites
But as they are spreading to torso and associated with itching, could an allergic skin reaction also a possibility.
You can apply a calamine
containing lotion externally over the skin for soothing effect.
Take oral antihistamine like cetirizine
to get relief from itching.
Consult the doctor for clinical examination and for further medication.
Hope I have answered your query.