If you are not on any medications like anticoagulants, blood thinners or birth control pills then this could be due to clotting disorders, low platelet count, fragile spider veins, deep vein thrombus, deficiency of Vitamin B12, folic acid, or Vit K, a liver disease or certain cancers. It could also be due to pressure urticaria. If you are on medications then you need to discuss that with your doctor. It could also be due to pressure urticaria. Any pressure, scratch etc results in hives. Keep your skin well hydrated, drink plenty of fluids, eat citrus fruits, use a good moisturizer and avoid scratching. If you feel the need to scratch, then use your finger pads to rub the area instead of the normal scratch.
On your legs, it can also be urticaria or hives. The swellings or wheals produced after itching can be pale (of skin color) or red. Urticaria is usually triggered by allergens, such as drugs, foods, insect bites, inhaled substances, in response to sunlight, cold, heat, stress, exercise, or a viral infection. Other common allergens are pollen, mold, dust, pet dander, or cosmetics. Since you experienced an itch, just go over these possibilities in your mind. Something may strike you. Over the counter antihistamines help relieve itch. Calamine lotions and other over the counter preparations with camphor/menthol do help to reduce the itch. Try a soft cotton ball dipped in cool water applied over the itchy areas instead.
Please consult a skin specialist regarding this.