Lindsay Lohan -- I Swear to God ... I Had a Miscarriage

Lindsay Lohan has sworn under penalty of perjury she suffered a miscarriage ... which means she better be telling the truth, or she could land in jail.

Lindsay made the declaration in new court docs in a $5 million lawsuit over her clothing line 6126.

You'll recall, Lindsay was given a reprieve from responding to the lawsuit while she was in rehab last year -- but even after getting out she failed to respond ... so she lost the case.

Lindsay wants to undo the damage and she's using her miscarriage as an excuse.

In the new court docs, Lindsay writes, "I have been overwhelmed since leaving rehab and dealing with my sobriety and a miscarriage."

There's been tons of speculation that Lindsay made up the miscarriage story to garner buzz for her Oprah show -- or maybe just to get sympathy votes -- but these are legal documents ... which means she's under oath.

If it comes out that Lindsay lied in the docs ... under penalty of perjury ... she could face jail time.