Bitter lettuce in April

I fixed a salad last night with the florellenschluss lettuce that's been going gangbusters in the cold frame. I've had the automatic arm to keep it from overheating during the day but with the weather warming up, it's still getting too warm in there and the lettuce started tasting bitter, particularly near the base. So today, I pulled off the cold frame to keep the temps down and hopefully extend the green season. The good news is that the cold frame is now covering two tomato and two pepper plants out in the garden with enough room still in between for me to use for hardening off seedlings.