Happens to me, when my nitrates get super high (from bio filtration, not dosed nitrates), the pH drops down.
Are you using a canister filter? If you are, and haven't invested a lot in biomedia, I suggest Seachem Matrix, which will harbor anaerobic bacteria (which eats nitrates). It won't lower them quickly, but over time the bacteria will eat through the nitrates.
The whole cycling process is a bit acidifying, which is good in a planted tank, but not as good in a african cichlid tank, haha