Lettuce is a cool weather crop. If the weather is hot, it can affect the flavor. Over fertilizing or using chemical fertilizers can result in a bitter taste. If the lettuce bolts (goes to seed) it will make the lettuce VERY bitter tasting.
The only one you listed that I can think of that could be the cause is over fertilizing, maybe. I used fish emultion once per month. I grew in cool weather, but we did have a really unusual warm break early on, maybe that's it. Thanks again.