Unfair billing

I Resigned from GEMS in 28/02/2015 of which I gave them a moths' notice. I was paying them through my employer of which they took their last payment in 15/02/2015. Now GEMS is sending me summonses demanding R2840. I can not pay for March because I was with another medical aid as of the 01/03/2015. My membership certificate shows that membership was terminated February of which GEMS cancelled a stop order with my employer then