In general, as long as you ensure that your product’s final pH is between 4.5 to 5.5, the lemon should not be causing issues, unless you are sensitive to this ingredient specifically. I'd recommend double-checking the pH of your shampoo A. It can also slightly vary over the course of the week after you make the product, depending on which bottle it’s preserved in. For future shampoo making, you can use a few drops of Apple Cider Vinegar (ACV) instead of lemon to adjust the final pH to see if it works better for you!