data bundles first usage

hi<br> <br> i received an sms saying i have run out of data so conttinued to purchase a bundle, however on further inspection online where i can view each bundle spearately i notice i still have MB remaining on a bundle which is set to expire at the end of july. i would like to know why it is that the bundle i have purchased which now expires in is depleting first instead of the bundle that will deplete in july. and if this can be changed so bundles deplete by expiry date.