My chilli plants have all of a sudden started to curl up and droop! I was wondering what's wrong?

Backgroud: They have had no issues in the past up to this point. The soil is not bone dry but is not soaking wet (moist). They were transplanted into these pots about 1.5 weeks ago and it's only started yesterday (and getting worse!). It's the same environment as before (windowledge) and the temprature has stayed about the same. The only possible thing I can think that's changed is the transplanting over a week ago, and that the plants have been watered only once and the soil is still moist? Too water retentive?

All 15 or so of my plants are doing exactly the same thing to some degree or another. What is causing it and how can I fix it? It's literally happened overnight.
Thanks all,