In general, employment in the state of Iowa is "at will" absent an express agreement to the contrary. At will employment can be terminated for any reason not amounting to discrimination on the basis of a legally protected trait (race, religion, gender, etc.) or retaliation for engaging in certain forms of legally protected conduct (filing a wage claim, taking FMLA leave, etc.). It doesn't matter whether the basis for termination is fair, reasonable or even true.
This unfortunately means that there is nothing illegal about terminating an employee because they missed a shift, even if the employee informed their employer that they were going to miss the shift and even if the employee did absolutely nothing wrong. Just as the employee can quit without cause or reason, the employer can terminate the employment relationship with the same freedom.
This is why I asked about ulterior motives. If you could prove that the stated reason for your termination was really just a fabricated pretense to termiante you because of a legally protected trait or activity (as defined above), you may have a claim. But this would take far more than a general suspicion that your employer may have been motivated by something else. You would have to allege a SPECIFIC illegal motive and you would have to have compelling evidence that this illegal motive was the true reason you were fired. It would not seem that you have this sort of evidence, as you indicate you do not know what your employer's ulterior motive might be. And if you can't prove an illegal ulterior motive, you regretfully would not have a claim due to the "at will" employment doctrine which otherwise permits your employer to fire you at will.
I hope that you find this information helpful and am genuinely sorry if it is not what you were hoping to hear. Please do not hesitate to let me know if you have any questions or concerns regarding the above and I will be more than happy to assist you further.
If you do not require any further assistance, please be so kind as to provide a positive rating of my service so that I may receive credit for assisting you. Very best wishes moving forward.