Why is the pH level of carbonation loss decreasing then increases afterwards? We have done an experiment of the titration of unsealed sprite soda to find the amount of carbonic acid being loss on various duration (0 to 1 hour), due to decrease pressure. 
The calculation was to determine the pH of carbonic acid overtime, thus the number of concentration of flat and unsealed soda are subtracted and is converted to pH level using -log[concentration of flat soda - unsealed soda].This was done multiple times on multiple duration until 1 hour. Now the results has demonstrated that carbonic acid pH level was decreasing in the first 30 minutes but then afterwards it was increasing. This contradicts on what my hypothesis is as it states that due to pressure being decreased (by unsealing the soda) the pH will increase turning more basic. Is this due to the factors of pressure and temperature? Can you please describe this thoroughly? Thank you for your time on reading this.
At the beginning, there was titrated mostly just the citric acid and the equilibrium amount of carbonic acid.

After that, carbon dioxide slowly converted to carbonic acid and $\mathrm{pH}$ has been decreasing.

Finally, the long term process took dominance as dissolved carbon dioxide together with carbonic acid were  leaving solution as gaseous carbon dioxide and $\mathrm{pH}$ have been raising.