CO2 will drop the pH as you surmised. To tell if that is the case take some water from the tank and let it sit for a few hours to let the CO2 offgas and test the water. It should test more alkaline.
FYI, once those sword plants get going they are going to outgrow your tank (if they are Amazon swords that is). They can get quite large when healthy and will dominate your 12 gallon. They are also heavy root feeders so unless you have a nutrient rich substrate I would recommend planting some root tabs for them. They are a great looking plant when healthy.