It sounds like what happened was that your landord actually interpreted your request to sublet the unit as one to rent as roommates, without understanding that you intended your mother to be on the lease as a co-signer or gurantor or whatever she was in the original lease. And then you signed the new lease without understanding what you were signing.
Besides omitting your mother, it doesn't sound like there are any difference between what you thought you were signing and what you actually signed, but in CA there's a rationale to "reform" (re-write) a contract to state what the parties intend based on a mutual mistake of fact, so you can ask your landlord about doing that.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
Disclaimer: The materials provided below are informational and should not be relied upon as legal advice.
If you were on a month-to-month tenancy, either you or your landlord could terminate the tenancy with a 30 day notice. Most written leases contain an integration clause that basically states the written agreement contains all that was agreed between the parties, and that the agreement supersedes are prior agreements. If your new lease contains such a clause, it effectively voids any prior lease/rental arrangements. If it was not the intent of the parties to void the pre-existing rental agreement, you should speak with your landlord and see if she/he is amenable to reversing this change. You should consult your own attorney to protect your legal rights.