Hello and welcome to healthcare magic .
From the brief history that you have provided ,I am of the opinion that you might be suffering from
Erythema Nodosum (EM ).
EN is an acute, nodular, reddish eruption that usually is limited to the extensor (shin )aspects of the lower limbs .
It is presumed to be a
hypersensitivity reaction and may occur in association with several systemic diseases or drug therapies, or it may be idiopathic.
Other diseases that may mimic this condition are acute
skin allergy (urticaria ),Skin infections (erysipelas ) and
vasculitis (inflammation of the blood vessels ).
This is a self limiting disease that can heal with in 3-6 weeks of appearing .
But depending on the causative factor , thorough investigations to find out the basic cause should be undertaken .I recommend visiting a
dermatologist, who after clinical examination might recommend the further diagnostic tests needed .