Your post is rather vague as to what, exactly, is different about your new position from your old one. So long as the decision to change your job duties was motivated by something other than your leave for pregnancy, yes, they can change your job duties when you come back to work.
I would like to know what they told you was the reason you would no longer have the same job. How many employees worked there? You should talk to a lawyer familiar with employment pregnancy discrimination claims. You have a very short window (six months from the date of the adverse treatment) to file a complaint with the U.S. EEOC. However, I believe you may want to consult with an attorney about filing a charge with the Ohio Civil Rights Commission. The Commission begins its process with first giving the parties an opportunity to mediate the dispute (what is it that you would like the employer to do, for example.)
I cannot give you a simple yes or no answer to your question without more details. Details that a consultation with an employment should cover. Many attorneys are willing to do free consultations, but always ask before scheduling a consult.