Problem, modal suddenly disapearring

Hi guys,

I have put the following code for modal in my page-contact.php.

<h2>Small Modal</h2>
  <!-- Trigger the modal with a button -->
  <button type="button" class="btn btn-info btn-lg" data-toggle="modal" data-target="#myModal">Open Small Modal</button>

  <!-- Modal -->
  <div class="modal fade" id="myModal" role="dialog">
    <div class="modal-dialog modal-sm">
      <div class="modal-content">
        <div class="modal-header">
          <button type="button" class="close" data-dismiss="modal">×</button>
          <h4 class="modal-title">Modal Header</h4>
        </div>
        <div class="modal-body">
          <p>This is a small modal.</p>
        </div>
        <div class="modal-footer">
          <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
        </div>
      </div>
    </div>
  </div>

After clicking the button open modal, modal is appearing, and suddenly disappears. Weird is also that in the right top angle I have an x for closing the modal.

Can someone advice?

Regards, Ivana