Were you working full-time prior to mat leave?
Are the other BSM's full-time?
What is considered full-time in your org?
Is the increase pro-rata - or was yours 2500 on top of normal fulltime wage or part-time wage?

Check these things out first hun.

Recently with performance pay for my level in my industry for performance of Excellent $6000 was given. Now even though no-one does my job when I am not there (29 hours per week) - my bonus was still pro-rata which thought unfair as it was a full-time position before I started, and a full-timer coming into replace me too But all legal

Good luck