I think we can rule out lack of oxygen. I'm not familiar with the 195 but used to operate the 135/145 and they had excellent pressurisation controllers and ear discomfort was never an issue even if you had a cold. Very occasionally the automatic system can fail and then the standby or manual system would be used as a back up. It's 100% safe but can cause ear discomfort if mis handled. If, for example, the crew were given a last minute descent clearance and were slightly late reconfiguring the pressurisation panel then you could have ear problems. I'm only speculating, of course, and am not criticising the crew.