pH swing question

So I started to mix nutrient solution 5 gallons at a time and probably use a gallon a day (coco). Usually the solution is ph'd down to about 5.9 and used immediately. I noticed this morning the pH rose to about 6.5, after the solution has been sitting in the bucket for a day with a circulation pump.

What gives? Will pH just constantly climb or did it just not "settle" after my initial mix? Do you guys usually have to pH again after a day of mixing your solution?

Thanks!