i will have a business trip to san Diego next month and wanted to pop into TJ
problem is my passport expired in june

will i be able to get back in with an expired US passport?

Yes.  You'll get a lecture, but afterwards you'll be allowed to enter the country.

Also, a passport is not required to enter the US from Tijuana.  You need:
--a gov't picture ID
--something that shows your right to live in the US

A drivers license and birth certificate is used by many people who don't have a passport.  They work fine.

i have no clue where my birth certificate is i should get a copy

Contact the county clerk in the county where you were born.  They'll either be able to provide a certified copy (not a run-of-the-mill xerox copy) or tell you how to get a certified copy of your birth certificate.