There are serious problems that can result in heavy bleeding or recurrent large clots. Keep in mind, though, that a large clot is only the sudden passage of a clump of partially clotted blood that's been collecting for some time. What you're seeing may be a whole night's collection, with sudden vertical position upon rising allowing gravity to cause it to fall out. But recurrent clots aren't normal. Also, the piece of placenta that caused the bleeding is probably washed away with the sudden bleeding episode, curing itself.
Inflammatory states can delay the normal healing and involution processes, resulting in bleeding.
In this case, antibiotics and medicines to contract the uterus are indicated. Also, clotting studies should be considered, since bleeding may be due to problems with the normal abilities of the blood to clot, and not because of any problems with normal involution. The typical puerperum is marked by lochia rubra for a few days, growing paler as the lochia alba by 10-14 days. Bleeding can occur off and on for up to six weeks. An occasional clot is forgivable, but recurrent clots or significant bleeding after the first few days should prompt an evaluation by your doctor.