It is fairly normal to get a few of the older lower leaves die and drop off even at this earlyish stage. My plants are really healthy but I have 3 lower leaves on each plant that look awful (yellow, brown spots etc..). In general old growth is old news, as long as new growth is unaffected it usually isn't an issue or a sign of any major problems. It would be nice if you could post a pic of whole plant though, just to make sure.

Also if you need re-assuring visit a few of the other grow logs, you will see plenty of pics of yellow leaves on fat budded plants.