It could be nitrogen deficiency. I think it's normal for the first leaves to eventually yellow and die off. Not sure if you are giving nutes yet. You could try a dilute (1/4 to 1/2 strength) dose of fertilizer. With nitrogen deficiency the yellow leaves don't ususally recover, but new growth will be healthy. Best of luck my friend.
true that, it is a small pot. the problem is nitrogen, that is why the leaf is yellowing. what you do about it is your decision, you can transplant, or add nutrients, but those leafs are pretty much done for they will yellow and die and fall off and the rest of the plant will be alive and growing strong.
Those are your Cotyldon leafs. When they turn yellow it means that they have used up all the nutes that the seed had to offer and is now ready for added nutes. 1/4 dose per gallon first 2 weeks. Transplant her into a 1 gallon first and keep her there until she looks like she want to bust out. Good for the roots to run out of room and thicken up. Then go 3 gallon till harvest.