I've had the same happen to me. For me it was a heat issue but a PH below 5.5 is bad regardless. I have to adjust PH every day in my tub for a few days after topping off till things level off then it's fine. I've had strains that won't take heat AT ALL. What your getting is a shock to the flux between your light temp and dark temp, a 20 degree swing is for sure going to make your plant look sick. 85 might be too high, try to get down to 75 - 80...add a cheap fan for the light.