How do I know if my termination was lawful?

Colorado is an at-will employment law state-- which means they don't even have to give you a reason to terminate you.  Either the employer or employee can choose to end the employment relationship at any time.  The exception to this rule is for employers who terminate employees for an illegal reason-- like racial discrimination, gender discrimination, or age discrimination.  By itself- what you describe would not be considered illegal discrimination, even though it is certainly chicken of your bosses to do.  (I'm trying to use nice language since this is a public forum.)  If there were one or two more factors-- then it could constitute a violation of state or federal laws.  Here are some examples of how the factors could affect your case:

1.  If other people complained about their schedules but were not terminated, but you were the only one that was.... and you happen to be a member of a protected class... then the focus on you could be a violation of state and federal discrimination laws.

2.  If you were going up the chain to raise concerns about working everyday because of the safety risk it imposed on workers to have absolutely no days off, then you could have a potential claim for raising OSHA safety issues and their retaliatory actions.

3.  If you were a member of a union or had been trying to help other employees organize to assert their labor rights, then this could be a violation of federal labor laws which prohibit discrimination against legitimate union activity.

4.  If you have recently had a workplace injury and you were they only complaining employee to be terminated-- then this could be considered a pretext excuse for worker's comp retaliation.

I don't mean to bombard you with a list of "what ifs", but I wanted to show you how one or two factors could change the scope and accountabilty of their actions.

From here, you have a couple of different options.  First one is to consult with an employment law attorney. They should go through each of the "what if's" I listed above to see if there was an unlawful motive to their actions.  If you believe that you have been discriminated against, you can file a complaint with the appropriate state or federal agency.  Colorado has a Civil Rights Division which is a good starting point for filing a complaint... and it's free.  A non-legal option would be to set up a meeting with the head of upper management and appeal your termination.  Not all companies offer this option-- but it doesn't hurt to ask.  Your plea should essentially be:  if a company retaliates against employees who ask for help, then it will serve as a significant demotivator for other employees to be dedicated to such a company-- thereby affecting productivity.  Many larger companies tout diversity training, inclusion training, and effective employee communications-- but what trickles to the lowest level of managers is that being a jerk is what gets you ahead in the company.  Upper management, if they are worth a hoot, will care that lower management has tossed the company values out the window. 

Another option for gathering information about a potential claim is to file for unemployment.  If they contest you receiving unemployment, they will have to provide a legitimate reason.  If that reason does not match what they told you initially, then the change in position would provide some indication that their motives were less than pure.

I wish you the best of luck... I hope this gave you some options to mull over.  Take a little time and look are your state's website as well.  They do offer some good additional resources.

You need to immediately contact your state's labor department, the Equal Employment Opportunity Commission and possibly private counsel.  This is not insubordination. You were simply asking for a resolution and this was not the manner in which it should have been handled. While most states do follow "at-will" employment, wherein you can be fired with or without cause, the fact of unlawful firing is still a pretty serious issue in every state. Do not wait on this; file immediately and while this is going on, consider whether you would or could qualify for unemployment.