The new conditioner might do it (it is an acid after all, esp. if double dosed.). Salt might do it too ... depending on how much. 0.9% salt solution has a pH of 5.5 .... but I doubt your level is that high (that is SW
But I bring up the salt because of salt creep .... if you add salt to change water to make up for both removed & evaporated water, your salt level will creep up (as evaporated water has no salt) ... but that will take some time ... prob. not in 2 weeks.
If you have been doing your regular water change, I don't think it is from your tank being established. In the case of Old tank syndrome, the pH drops (or crashes) because the buffers are used up (fish poop, decaying matter, etc. all produce acids & consume buffers). But if you are doing regular water changes, the buffers should be well replenished & should not give you pH problems.