Problem with retainer?

Mine did exactly the same thing at first, its because your teeth have had no support from the monday to the wednesday and so have been given chance to move. Put the retainer on and try to keep it held down so that the teeth on your lower jaw can become re-accustomed to the position that they were in when you had the braces taken off. Sometimes if I haven't worn my retainer for a few days it springs up from my bottom row of teeth so I have to put it on and wear it overnight and then in the morning it fits snugly. Just make sure you wear it tonight and tomorrow and if the situation hasn't changed then go back to your Dentist and ask them for a re-fitting. The only thing I'd say about the refitting is that you will still have to wait another few days before the second retainer will come through, by which time your teeth will have moved even more, so I'd stick with your first one if you can!