Lettuce tastes either good,or weird/bad at times

[LittleSquirrel]

I've only bought organic lettuce for a long time so I'm not sure if this is true with the non organic, and I ususally buy it loose (not in plastic container). Lately I eat a lot of green/red leaf lettuce in salad. I can't figure out why sometimes the lettuce will taste good and normal, but other times it will taste strange or have a weird sweet taste or other times be bitter, enough to make me not want it again. Is it just different growing conditions? Does anybody know when green/red leaf and romaine lettuce are in peak season? Another question - are baby greens better for you than the mature ones?