It could be an insect bite. If the lump is still there, better to apply some ointment (steroids
0.1% beclomethasone) and watch for any regression or increase in it's size. sometime it can cause severe infection of the skin, rather below the upper layer of the skin called cellulitis
which need good effective treatment, like intravenous antibiotics.
Better to get it checked out by a doctor as soon as possible if it continue to bother you.