How are you testing chlorine? If you are using DPD testing (comaprator with shades of red) or strips be aware that these can bleach out at high chlorine levels leading you to think the chlorine is much lower than it really is.
If this is the case it could explain the pH spike.

Also, what is your TA. High TA will cause the pH to rise. You did not state how often you are testing pH. If it is not daily (which it really should be) then your pH might not have 'spiked' but rather just climbed over time because of the TA.

Posting a full set of test results not done with strips would be very helpful, as would a discription of your pool and equipment, including any water features or other potential aerators of the water. Aeration of the water will cause pH to rise (and we use this fact to our advantage when we lower TA).