Why is my data gone?

I am not normally someone that complains, but I am rather dissapointed in Mtn. I upgraded a few months ago, but my new contract only started yesterday. My previous contract I did not receive data, so I had to buy data out of my own pocket. On monday, I bought myself, out of my own pocket 750 mb of data. On Tuesday, my contract migrated. Mtn took all my airtime and smses that was stil on my phone, I understand that because it was part of my contract, but then, they also took my data that I bought with my own money. I went to ask why this will happen, and the answer I got was that that is just what happpens. I cant believe they are allowed to do that, if they do this with a few people in a week, imagine how much money they take. Very dissapointed, and would like my data back.