flushed, ph reading 3.5 to 4.0???????

Hello fellow tokers, why would my ph be so low after a flush? I am using good soil, water was ph'd at 6.8, soil is super wet and checked ph and it is through the acidic roof??? I am growing in a 5 gallon bucket, flushed with at least ten gallons of h2o??? I was using a 12-23-10 nutrient solution for flower prior to flush, and used it sparingly. The plant is Indica dominant, bagseed, and the buds are healthy looking. Could it be my cheap ph meter bought at a local grow store?? The plant is 5 feet tall with at least two to three ounces of wet bud on her!! She was not yellowing at all during flower since my nutes contained nitrogen. I have checked ph before and it was between 5.5 to 6.8, varied in different depths of soil. So any ideas why so acidic just after a flush??

Bump, bump, bump, bump it up, cmon fellow tokers

bump me to the top, bump me to the top, someone have an answer??

sometimes i need to clean the probes of my ph meter with scotchbrite or emery cloth. if thats not it, keep flushing with 2 or 3 gallons of clean water to each gallon of growing medium.