Thank you for the information and your question. First, this request or action would not be "unconstitutional." There is nothing in the constitution that prohibits this. However, it may or may not be unlawful. That though could only be decided by the EEOC or courts once they have considered all of the evidence submitted from all parties, which I don't have. I can though discuss the legal issue that is potentially present.
Normally, an employer, especially a bank because of stricter employment in banking laws, can run background checks on their employees at any time during their employment. However, the issue arises as to who and why they are asking for the check. In this case, you mention that you are returning from medical leave. If the employer only does background checks on individuals returning from medical leave, or only on you because of your gender, race, national origin, age (over 40), disability, religion, genetic information, pregnancy, or military service, that would be unlawful as violating Federal discrimination laws and you could have a claim to file with the EEOC and eventually the courts.
If you believe you are being targeting only because of the limited reasons I mentioned above, you should file a discrimination complaint with your HR Department and if they do not resolve the issue in your favor, you can then file a discrimination complaint with the EEOC and might also want to sit down with a local employment law attorney to discuss your case.
Please feel free to ask for clarification if needed. If none is needed, then if you could take a moment to leave a positive rating in the box above, I will receive credit for assisting you today. Thank you