What causes multiple reddish brown ovals, mildly indented, on the tibia?

?Erythema Nodosum: While there are certainly other causes, en is a common hypersensitivity (allergic reaction of sorts) reaction commonly found on the shins, calves and buttocks. Often fever and joint aches occur. If it's en the lesions usually start in clusters and are tender, then after a few days turn into painful nodules and then bruising. Symptoms can be treated unless an underlying cause is found.