Bargaining my own shifts

Hi all! I am a regular PRN nurse who is leaving my current job to be relocated. Here is the issue I'm trying to wrap my head around in trying to understand. As a plane PRN, I am technically doesn't required to be on the working schedule only if I wanted to but I informed my manager in advance when I took this position that I can work certain (fixed) amount of shifts per month therefore she can ago ahead schedule me on any day as long as with the fixed number. Now that I am leaving and giving sufficient notice (over a month) I am being scheduled to work extra days more that I had original agreed too. Her reasoning is because it's within my notice day although I had already fulfilled the shifts required of that month. Lots of it has to do with short staffing. Anyway, I do not want to burn bridges so I told her I can only work some of the shifts, not all of them because I have real reasoning to why I cannot work. So, is this normal? If it was you how would you deal with it? The problem is we always short staff and that makes me feel really guilty and most of the time I cannot say no. However, my manager has been so nice though and always gave me the days I needed off when I was not PRN. The thing too was that I always worked whatever the days she scheduled me to work and I only had called in sick once during my employment there. Never complaint much about scheduling as much as other staff who I always heard speaking of every time I am at work.