Hi Diamond...Citrus are sun-loving plants, but like most tropicals need acclimation to higher light levels..
What type of soil do you have lemon planted in? Is it well-draining? Also, how tall is your tree? When was the last time it was repotted? If it's rootbound, it'll dry out much faster. Citrus love the great outdoors in summer..For the time being, do you have a spot on the roof w/some shade or is it all sun? If the Meyer is smaller than another plant, can you set behind for shade?
When you went to water today, was the soil dry? Perhaps it only needed a hearty drink. When you water, do you see water seeping out of drainage holes? This indicates the plant is well-watered.
I'd take back out, perhaps on a cloudy day, say when they predict rain for the week. I find night or early morning is the best time. So, don't set out on a sunny day.
I've had cactus sunburn, placed in western exposure, and they live in south windows all winter long..And we know how much light cactus need. (S)
Keep us posted how it works out..Toni

Are your pots plastic or clay (glazed or unglazed)? Are the plants rootbound? Is your rooftop covered with hot black asphalt? 6 and 8 inch pots are pretty hard to keep watered in the heat of the summer in full sun outdoors, especially if they are rootbound, even if you're not on a roof (I have some on my deck that challenge me).