Over watering and under watering is often a problem in container growing. One cubic foot of soil is almost 7 gallons. Probably on the lower side for Super Sweet 100 which is a vigorous variety. If your container has more room ad some more soil to it.

What happens is that frequent watering can/may wash down and out the nutrients. For this reason you have to fertilize 3 times as often but at 1/3 recommended strength.

Another thing is that as a plant grows in restricted soil volume it will abort the lower leaves . That even happens in the garden bed. I would just prune them at that point.

Sey

It could be Magnesium deficiency.

What you can try is to dissolve about one Tbs of Epsom Salt in a gallon and water your plant with couple of times ( 5 days apart).

Or look into foliar spraying with it for immediate result. I don't know the amount for spraying. Look it up.

Wait for more comments coming your way.

Sey

Look for spider mites under those leaves. A possibility.