We hand brew our probiotic and so each of our brews does have its own personality. This means that different batches may taste different and may, as you say, become a little fizzy. The fizz is the result of the probiotic going into what is called secondary fermentation. This means that some fermentation has taken place in the bottle, and the gas that is a by-product of fermentation dissolves back into the liquid causing it to be a little fizzy.
Secondary fermentation indicates that the bacteria are alive and active so the product certainly does still work and is perfectly safe to drink.