Hey mate, if you are growing organically in compost it will buffer to some extent, but when you add non-organic fertiliser that can cause nutrient/salt buildup, which in turn will effect the PH of the medium, it could have possibly been die back though. I'd recommend you get a cheap PH pen from Amazon, they are really handy to have. It gives you the ability to rule things out if you do come into an issue, if you dont test you wont know for certain.
Hope this helps 🙂