What you added in distilled water did little to change mix - no worries.
Losing coolant though is an issue b/c it is a CLOSED pressurized system. Coolant is only lost when a part of the system is compromised: radiator, head gasket, tank cap, relief valve, oil cooler, water pump, coolant reservoir, etc. Now, maybe when you had H2O pump changed they let a large bubble in system which finally burped itself out. Doubtful but how long ago was pump changed? Otherwise, have a quick pressure test done on the system to see if it holds and go from there...
Good luck
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