Unfortunately, you have no rights in this matter. As an "at will" worker you could have been fired for any reason or no reason at all, with or without notice. Further, you were aware that you were in a probationary period and that this could happen. The fact is that a company can set the conditions of employment much as it sees fit (absent some form of legally actionable discrimination). Accordingly, unless you had protection under the terms of a union agreemnt or employment contract, you have no claim here.