Sorry to read about this.
What percentage of the filter media did you change? If less than a third, for example, it's unlikely that this is the cause - it ought to be quite easy to recoup the good bacteria over the next few weeks unless you're significantly overfeeding.
Presumably you didn't do anything else like run the filter media under tap water (rather than discarded tank water)?
Do you know what your GH and KH / water hardness readings are? If they're very low, then that may account for a PH crash.
Is there anything else which you've added to the tank or done differently?
What is your tap level PH reading normally?
What are the ammonia/nitrite/nitrate readings?
Sorry for all the questions. I'm tempted to suggest a water change, as this ordinarily helps problems, but it might not necessarily be the best course of action, hence all the questions. We'll do our best to help once you get back to us on these questions.