I have had that exact same problem with seedlings. I recently found out that it is in need of some (K) Potasium and at that young stage they probs need a good N P K in a tiny amount.

just what I have found out though. I don't know too too much. But In my last 3 grows I have finally learned that. red Stems and veins are bad...

And some strains handle it better than others.

Check the Plant Problems for the charts people have been posting there is one with an exact picture of what your seedling looks like. Droooped curled leaves and red stems etc.

kd