Why does my ph drop overnight?

New in the indoor sector, I have a Northern Lights in a 5 gal bubbler. She's several weeks into veg, and growing like, well, like a weed. Very healthy, massive white rootball, and is now drinking up a lot of water each day.
Yesterday morning I checked the ph and it had fallen from 5.8 the morning before, to 5.4, so I didn't get too excited. Just brought it back up 5.8, no problem.
This morning, it was back down to 4.7, this got me concerned. Brought it back up to 5.8
I have 2 other buckets going with tai stick, that are about 2-3 weeks behind this one, same set up and the ph has remained stable thus far.
I'm using- distilled water
DynaGrow Foliage pro
DynaGro Protect
General Hydroponics PH Down
API PH Up
400w mh
The Northern Lights has, in the last week or so, has really begun to suck up the water and nutes. Is that what's causing this?
Can anyone tell me more about these wild fluctuations?

are you sure she is sucking up the nutes? if the plant sucks up just water then your ph will go down because the concentration of nutes rises. check the ppm when this is happening.

Yeah, she seems to be sucking up the nutes, too. My starting ppm yesterday was 465, and it was down to 420 this morning at the same time. Being new to this, I check readings every morning at 5 when my light kicks on, just so I can get a grip on how it responds and reacts on a day by day basis. Don't know if this is good or not, but I enjoy it.She appears to be using about 1 to 1-1/2 pints of water a day. I'll be mixing more solution maybe tonight to add in tomorrow, probably. Before, I would mix it then add, but I may try mixing it tonight, let it stabilize over night, then re-check it in the mornig, and maybe I can at least isolate which process might be causing it to fluctuate?
Could it suck up just water, no nutes, and still be as pretty and healthy as she is?
I'll see about getting some pics, if you think it might help.
I will definitely be monitoring her water/nutes intake even more closely, now that you've brought it up, but so far haven't found anything outta line- for what little I know about hydro. This one's my cherry- literally. So I really appreciate all the help I can get.
Thanks!

I had this problem and it was the start of root rot. Now I know everyone's situation is different but are you using something like hygrozyme to help eat away the dead roots?

Also what about the temp. of your res. is it below 70F? When my res. temps went up I had all kinds of wacky PH issues.

Just some advice from over here so take it as just that some advice and do what works for you and your system. Good luck!