Can an employer withold a discretionary bonus b/c you were on maternity when the bonus was awarded?

My employer has a discretionary bonus program that when we work there for 18 months we enter a pool and receive a bonus at the end of each quarter if the company is doing well. This program is not in the employee hand book. I went on maternity leave in early March and will be back at work on May 1. My employer awarded the Q1 bonus to eligible employees on April 12. I had received a small paycheck that day for working a few hours from home while on maternity leave but did not receive the bonus. When I inquired about to my boss he stated because I am on leave I don't qualify. Is this legal or discrimination? Also, after I inquired my boss about this he sent out an email to all other employees stating not to talk about the bonus pool or he will take it away. Something doesn't seem right here.