The leaves on my plant are a very dark thin colour...

Yes, way over fed, and yes, this is a bad thing. The uber green leaves and curling/clawing of the leaves is a sign of nitrogen toxicity and burned tips are sure to appear soon if you keep up with the over feeding. Thin/narrow leaves are not an issue. In soil it is a schedule of plain water/plain water/feed/plain water/plain water/feed.........so that the plants are only being fed every 10-14 days, which is plenty, especially for an auto flower.

It would be more than helpful if you had a diary for this grow. Without the information contained in a diary, all you're going to get here are guesses - and the guesses might be right but they might also be wrong. Please start a diary - we'd like to help and that would help us help you!

leaf dark, so nitrogen toxicity is possible my though it , excess of nitrogen blocked a bit absorption of calcium and with low humidity and high temps you have those first big signs off worry make few plain water cycles with right ph, letting her consume most of moist in the pot and when its easy to lift - only then - you water her. give her better conditions a bit, plan the support of branches for them not to waste energy and put everything to buds maybe for 3-4 days raise your light 10 cm higher Happy Growing !!!