Soft Bathroom Floor - leak or just poorly laid wood flooring

If the area you are relating to is on the second floor of your property I would be almost certain it's only due to being an old property with old flooring over a period of time the flooring has flexed therefor causing the slight movement you can feel in that area. If there was a leak there would be evidence to suggest so such as a patch on your ceiling. Or even damp on the out side wall. If you see any of those do call a plumber in to take a look for you. I hope this helps.

The rubber seal inside the large pipe round the waste can sometimes get brittle with age and start leaking slightly every time you flush. This could slowly be causing damage even over several years without being noticed. Wrap some kitchen roll round the waste and flush a few times to see if there are signs of a leak.