If you're using drop-based testing for your pH, then I would go ahead and lower it back to the mid-7's range with either muriatic acid or dry acid (but read the muriatic acid link in my sig first--handling it safely is crucial!). If you're using strips, then please follow Watermom's recommendation to get a good drop-based kit.

If you have your returns pointed upwards so that they ripple the water, that could cause aeration in the pool, and would cause pH to rise. A good hard rain will also aerate the water and cause the rise (as will splashing kids )

What is your TA? If it's not high enough, then you might have a hard time keeping your pH stable. TA should be in the 70-90 range if you're using bleach as your chlorination method.

Janet