Changed Maternity Leave

I just found out that my company changed its maternity leave policy while I was on leave. I had my baby a week before the new policy was implemented. They’re offering six fully paid weeks in addition to short term disability and FMLA. HR told me that when I gave birth I was only eligible for FMLA and short term disability so I can’t qualify for the new policy even though I was on leave when it took place. I’m so upset. I could’ve gotten six more weeks with my son. The least they could do is pay me...