Am I protected under FMLA law so that when I return to work I am making a similar amount to that which I was making before my leave?

When I started this job 1 year ago, I was paid a base salary until my 15% commission exceeded my base at which point I was moved to 100% commission pay structure. The girl who is covering for me during maternity leave is being paid a base salary until her commissions exceed that base and will continue to receive commission when I return for ads that were billed during my leave. 2 other sales reps hired within the last 12 months within the company were both paid a base salary until their commissions exceeded their base pay at which point they were put on 100% commission. While on maternity leave I will receive commission for ads that were billed up until the date of my departure so long as the money is collected within 90 days of when the ad sold. When I return from FMLA leave after having a baby, I was told that I will not be offered a base which means that it will take approx 90 days before I start making the same pay that I am making now