Based on the facts you describe, it sounds like you have a good claim for discrimination. The law provides you protection since there are 6 employees and where they are replacing you with someone as opposed to eliminating or reducing your position, you have stated facts that give you a claim. You should consult with an employment lawyer to pursue this.
How long were you out for? Were you planning on returning on the date you had agreed to return. You likely have a claim but you need to discuss this with an employment attorney.
Your employer may have violated the Massachusetts Maternity Leave Act.
You have 300 days from the date of termination to file a complaint with the Massachusetts Commission Against Discrimination, however, it is best that you act quickly. Find a local employment attorney who can review the circumstances of your employment, leave and termination and advise you about your best course of action.
You can find an attorney by searching Avvo or contact the Massachusetts Employment Lawyers Association for a referral.
This answer is provided for guidance only. DO NOT rely on it as legal advice. We DO NOT have an attorney-client relationship. You should contact an attorney in your area for a one-on-one consultation before pursuing any action or making any decisions.