Lawyers, listen up! (question about laws for hours worked in CA)

Well a while back, I was scheduled one day to work from something like 12AM-4AM (doing inv.), and then I worked the next morning like at 10 am or something. I had heard that you had to have 8 hrs between shifts, or you get overtime for everything afterwards. I got my check, no overtime. I questioned it, the boss and the District Leader or resource manager or whoever he talked to both said the law had changed. I tried to find stuff online, but it’s tough when you have limited knowledge of legal stuff anyway.
Well I gave up and some time passed, until yesterday when talking to my Assistant Manager and another emplyee and the issue came up. Well I said something to the effect that it was good that the other emp didnt have to do inv and then open, and they both said that was illegal cause of the 8 hr issue. So I’m wondering… anyone know the law in CA for this? If this is illegal, anyone think I have a case against the store? I wanna bring this motha down. I really hate it there, and this would be perfect.
The only problem is, once I couldn’t find info about the issue and both my Manager and resource manager said it was changed, I signed the lil paper we sign and cashed my check so I think it voids the issue. But I think if I took it to court, seeing that they told me otherwise and I have no way of knowing, and am young, maybe I would have something? I really didn’t wanna sign but didn’t have forever to debate the issue when the check was money waiting for me and I had no one to tell me officially besides my manager.