PRN Nurse Hours Reduced?

[KCase, ASN]

So I've been working PRN in an outpatient oncology clinic in Florida for three years now. And I absolutely love my job. I started out full time but became pregnant with my oldest daughter pretty early into starting and when I came back from maternity leave initially, I needed the flexibility. 
 

So I've been PRN for three years now. First of all, we are always short staffed and crazy busy. I'm one of three PRN's and two of us typically get fairly regular hours. There is always a need for us so that's not the issue here. I usually work four days a week, 8 hour shifts(9-5) with weekends off. 

There has never been an issue at all until now with me getting hours.
 

Today my manager informed me that corporate is saying that the PRN nurses can't work more than 19 hours a week(with 8 hour shifts, that's barely 2 1/2 days). That has never been something that has ever come up in my 3 years of being PRN until now. I do not ever remember anything ever being said about there being a maximum amount of hours I could work until now. It's very suspicious.
 

And basically I'm being told that even if there is a need in the clinic for me and even if I'm available they will reduce my hours or I was informed that I could go part time(I lose the flexibility of PRN, I lose the extra pay, I still have no benefits, and it's 24 hours a week and if I need the extra hours and money, I'm screwed). I really don't like basically having my job threatened or hours threatened. I understand PRN nurses aren't guaranteed hours but that's not the issue. They have a need and I have availability but the corporate office is saying they can't work us that many hours. Nothing like that was ever said to me three years ago and now it's being changed. 
 

Can they do this? Any advice?