Sources tell TMZ that Britney's doctors have two operating theories -- either that she suffers from post-partum depression or bipolar disorder. The doctors strongly believe post-partum is the problem.
As for substance abuse, as one source says, "No doubt about it - she likes to drink." But doctors believe the drinking is a way Spears has coped with a bigger problem.
We're told Britney, who is sticking it out at the Promises rehab facility in Malibu, is currently reading ...