On my shin i have discoloration much like a bruise, and a small lump leftover from a severe car wreck three months ago. what is a possible cause?

Hematoma: It is common to form hematomas over the shin/tibia after direct trauma. This collection of blood will gradually resorb, but it's common for a lump to exist for many months. If not painful and only slightly tender, it's often fine to observe it. If enlarging, painful or not resolving, then see a physician. If I can help, join my care team and virtual practice at www.healthtap.com/dr-ClarkeHolmes