Hey there,
I'm doing my first grow too and experienced the exact same problem as you. As a matter of fact, to a certain extent I am still experiencing it! I did a lot of searching and asked on this board and it seems the general consensus is that it's overwatered. I am guilty of over-loving my plants. It is amazing just how easy it is to overwater. I recently received a pH/Moisture meter and just earlier this afternoon I tested the plants....it's still on the upper border of moist and wet, even though the top of the soil is practically bone dry. Be very careful with this. I'm hoping this is the last time I'll do it now that I have a meter to give me an idea of the soil's state, but damn it's hard to sit back sometimes and let them be
Oh, one more thing, you'll be amazed at how soon those babies are ready to transplant. When I was researching the problem with my plants, they were a little bit bigger than that and they were already pretty clearly rootbound. So prepare to transplant into something bigger pretty soon!
Good luck, and send updates!