Sounds like you know what the answer is :) PH and overwatering. You don't need to be Uber accurate when working in soil because the soil will buffer thing to a degree unless you start adding strong acid etc.
The whole issue with litmus test are you cannot get a true reading unless you're in natural light. They plenty accurate enough to work with soil and probably more accurate than those cheap meters mate.
Personally I would give them a light feed and lay off the PH for a while. Also let the soil air a bit before feeding again. These will pick up lovely as soon as you make the required adjustments so no need to panic.
Good luck