Pool water feels unusal ???

I'm taking a vacation and have been swimming in a larger pool (50m) the last few days. I had swum in that same pool before on my other vacations and didn't sense anything unusual. But this time it feels different. Somehow, the water feels "heavy" and hard to pull, and it's difficult to control the body position while swimming (e.g. the body tilted in one direction and it's hard to correct it). At first I thought it was because I had not swum for a week and needed time to get "used" to it again, but the two people who went swimming with me, one of whom had been swimming in that pool often, said they had the same unusual feeling about the water, that it feels somehow "harder" and they got tired sooner than before. What could be the cause? Even if they had adjusted the chemicals in the pool differently than usual, it's hard to believe it could cause such dramatic effect. This pool is supposed to be ozone disinfected. I've never experienced the same before. And I'm certain now it's not something to do with my own swimming. Anyone can shed some light on this mystery? :confused::confused::confused: (I wonder if this might be close to what swimming in mercury feels like :rolleyes:)