Mickchick, are these Michelin tubes? Some tubes have a removable valve core. If this comes out a bit too much you could experience this same result.

If the tubes have removable cores, go ahead and pull it all the way out. Put a teensy bit of blue Loc-tite on the threads and replace it. Do the same to your spare tube. You will be happy you did. Youcould be having a very slow leak as we type.

SK who has learned from experience.