What's in the soil?
Looks like she may not have found enough Mg or K yet, but will probably grow out of it
I do sometimes give a little organic gro/root juice to help young plants get established, that might help
Looking good and it should not be all that hard to fix the plant and having it looking good again.
Is the plant a photo-period or an autoflower. Knowing which it is can help determine if anything that is going wrong needs immediate help or can be worked out over a couple of weeks.
Is that small plant growing in a natural soil or a soil-less mix for starters.
Then we need to know what fertilizers have been used so far, if any. And, if you are using something tell us about it and how much and how often.