I am not an expert, but it's always easier to tell with some pictures. Aslo, what strain is it, what soil are you using, any fertilizers? It might be nitrogen deficity so you would need to give her some nitrogen rich nutrients. If tips started to yellow up first it might be nute burn, therefore I would advise to flush.
As well as pics, the more info you can provide us, the better your answer/s will be.
Without knowing anything, I'll take a wild guess at over-watering; of course it could also be lack of light, nute deficiency, incorrect ph issue and a host of others.