What can cause serious back pain?

Several things: There are numerous causes of back pain, way too numerous to mention. See the following link fro WebMD regarding back pain: http://www.webmd.com/back-pain/guide/causes-of-back-pain

Back pain: Strain is a common cause of back pain but other causes include ruptured disk, fractured vertebra, malignancy,aneurysm,infection,pleurisy,pneumonia,etc. A good history and physical examination by your physician usually identifies the cause.

Many things: Muscle sprains, ligamentous injury, bone and disk injury, internal bleeding, blood vessel dissection, kidney infection, tumors, infection of the spine, spine or rib fractures, occasionally labor, pregnancy, and kidney stones. Sometimes cancer involving the bones of the spine and sacrum or other internal organs. As you can see the list is huge. More information would be helpful in narrowing it down