That could be the case, sounds like you might have some salts built up in the soil from overfeeding. I've been there
A flush is not the best thing to be doing right now but it is necessary. Keep it running off until your runoff falls within your desired range, it will take A LOT of water. The general rule is 3x the capacity of your container. I had PH 5.5 on my last grow from buildups and my PH meter falling out of calibration... Had to flush midway through flowering. It definitely effected my yield but it still turned out to be top notch smoke, just not as much of it. When you get a chance grab some PH calibration solution and check your meter every week or two.
I got lazy and that's mostly where my PH issues came from. Good luck, let us know how it works out.