I've had a similar thing happen with my first 2 grows but so far they grow out of it. I think in my case its a light intensity problem but I would go through and check the ph of any runoff you get. Your not adding any nutes yet right? if its not light intensity, heat, or ph the soil might have a bit too much nutrients for plants that young
Dunno if that helps but that's what my approach would be. If it gets worse perhaps reduce your lighting to 20 or 18 hours that seemed to help my last grow at one point during veg