[Mullins]

I don't think it's a great idea to dive FRC style (i.e. sink from the surface) but with full or partly full lungs. The whole point of FRC (which stands for Functional Residual Capacity, i.e. a passive exhale) is that you can afford to sink from the surface because you will have only a small buoyancy change on the way down. This is because there is not much gas in your lungs to compress. So, despite the free ride downwards you won't get a great deal heavier from about 10m onwards and the ascent won't be too difficult. However, if you take the same approach with inhale diving, then you will find yourself very heavy on the bottom because of the bigger volume change and the extra weight you needed in order to sink that early. Due to higher lung volume you also won't get the benefits of FRC (early & strong dive reflex, big blood shift) so you're getting the worst of both worlds.

The FRC gurus out there may have something to add but I would think you're better off doing proper FRC (passive exhale) dives to shallower depths rather than trying to combine the two dive styles and immediately hit your inhale depths. The symptoms you're getting definitely sound like they're pressure-related so you may have been exhaling more than you thought on at least one of those dives.

How deep do you go on inhale, and where are you neutral?