The most obvious reason why your pH could be low is a low KH, aka alkanity, aka buffering capasity. Basically, how many chemicals are there in the water that will help keep the pH stable.
To complicate things, your tank is currently cycling, or going through the process where bacteria is built up to handle the waste (ammonia and nitrite). It is very possible your pH will be a little higher after you are done cycling the tank.
For right now, begin doing daily water changes of 20-25%, as that will help remove some of the waste. You may want to begin wiht twice daily until your ammonia and nitrite are both below .5
What fish do you have in the tank?