Lower Leaf Problem

Hello! I'm new here and this is my first actual plant I've tried to grow. It was grown indoors at first, then brought out doors. It's in five gallons of good, composted soil wih added nutrients. Well everything was going fine until recently, the bottoms leaves have started to get ugly. The top leaves are perfect and still growing fine. I'm just wondering what could be the issue, is it nutrients, too much water, not enough water, too much sun, bugs, fungus, or is it just normal leaf stuff?

PS: any other tips and whatnot appreciated as well.

looks like something is sucking the sap outta those leaves

Look at the base of the stock. You may see some scale inscets on it. They dont move but do suck the sap out.
I had this problem with a few plants as well as my citrus tree.

What would be the best cure for this? Natural preferably.

Thrip damage. Monterray Garden Insect Spray (spinosad).