Hey man! Thanks for the answer! Normally I just use bottled mineral water from and have just fed this girl once (and with 1/3 of recommended dose). Maybe the overwatering has caused a lockout or deep problems in the roots?
Disclaimer: I would like to say that I am a complete beginner with almost no experience growing Cannabis.
This girl is an Auto Super Skunk (Geaseeds) and was put in 1L Airpot (with Biobizz Light Mix soil) after seed germination and she was going very well for around 3-4 weeks.