Where are they? It could be the cold nights. The other thing could be more serious. Is the compost you potted them into different from the one you sowed in? The have been a number of cases of compost contaminated with weedkiller I hope it's not that.
Where are you, when you say heat at night what temperature? Mine have gone from the kitchen window sill to the conservatory,I find they sulk and look like that for a few days after being potted on.I live in SE UK only 10 minutes from the sea,and it will be at least another month before they see the inside of a green house
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Overnight temps need to be at last 12C to keep them happy
Billericay - Essex
Knowledge is knowing that a tomato is a fruit.
Wisdom is not putting it in a fruit salad.