Was that the actual reason they gave you? If so then you may have a claim under the Pregnancy Discrimination Act and the similar state law. Talk to an employment lawyer in your area about your situation.
The short answer is probably not. The only type of discrimination that can get them in legal trouble is discrimating based on race, religion or gender. Other than that, they are legally allowed to hire (or not hire) whomever they choose for pretty much any reason.
Hope that information helps, best of luck to you.
The sort answer is "yes, you can sue." The more relevant question is whether you can win that suit, and the overwhelmingly likely answer to that question is "no." Generally, a business does not have to hire anyone, and can base that decision on any but a very few legal exceptions, such as race, religion, gender, age (i.e., "too old"), etc. Based on the facts you provided, I could expect that Little Caesar's would not want the risk of some issue arising between you and your child's father that could lead to a workplace disturbance.
Please remember, however, that specific questions entail specific facts as to which an experienced lawyer can give you reliable advice. Both my and other lawyers' answers to questions on Avvo are for general purposes only and do not establish an attorney-client relationship; nor should they be considered legal advice. They also are simply responses to the questions that are posed, and do not necessarily address every circumstance in your particular situation. Your best approach may be to contact a lawyer directly through Avvo, and have a consultation at which time you can provide complete details.