I bought leaf lettuce starts and planted them a couple weeks ago. They didn't grow much more than what they were when I bought them and now they are beginning to bolt. Temperatures have not been hot yet.
Bolting Lettuce Starts
Certified GKH Gardening Expert
This article will help you:
A plant "bolts" when it is under stress. It is caused by high temperatures, long periods of high light intensities, and drought. Could this possibly be water stress (under-watered)? How were you watering your lettuce?