5.4 isn't bad 5.5-6.0 is ideal, and what day are you in on, the plant it dosnt look likes its begun flowering yet, im pretty positive its nute burn, lowering its nutes for a few days letting it get a little bigger wont hurt it any, your still using veg nutes right? a plant that tall i would give whatever the one week veg is 400ppm should not be hurting it. but all strains are different with how much nutes they can handle, like fruity thai can handle a 2,500ppm feeding, most plants cant do that.
what formula are you using for your nutes?
P.s. if you cant afford a dehumidifier add another fan or two to your room, not directly blowing on the plant, it will lower your humidity.
MJ ideally grows at 40% humidity and 73 degrees Fahrenheit. Again this is ideal, not a requirment, I've seen plants grow bountiful and potent just fine at 70% humidity.