Attach a modal to the cancel button..
<form action="/submit/form/" method="POST">
<input name="text" placeholder="Email please">
<input name="" type="submit" class="btn btn-large big_btn " value="Save Changes">
<input href="#modal-dialog" data-toggle="modal" id="cancel_profile_changes" type="button" class="btn btn-large big_blkbtn hide_margtp_35_a" value="Cancel">
</form>
<div id="modal-dialog" class="modal">
<div class="modal-dialog">
<div class="modal-content">
<div class="modal-header">
<a href="#" data-dismiss="modal" aria-hidden="true" class="close">×</a>
<h3>Are you sure</h3>
</div>
<div class="modal-body">
<p>Do you want to discard changes?</p>
</div>
<div class="modal-footer">
<a href="#" data-dismiss="modal" id="btnConfirm" class="btn confirm">OK</a>
<a href="#" data-dismiss="modal" aria-hidden="true" class="btn secondary">Cancel</a>
</div>
</div>
</div>
</div>
Demo: http://bootply.com/93435