Well, that would be my guess. There's only three things a plant needs to live: water, air, & nutrients. It doesn't look like you're overwatering since your soil seems dry and it sounds like you have good ventillation and your temps aren't out of control, so I'm thinking it's something nutrient related. Your plants are now going from seedling stage into veg stage and require more nutrients, which they can now uptake since they now have a developed root system, to support all the new growth they're about to throw out. Don't be afraid to give them a LIGHT dose of nutes and gradually increase to the levels you want to be at.