Some sort of problem - Sorry no pics will give as much info as possible.

Heya, 2nd time grower here, Got a baby outside right now in a 10 gallon pot, she stands 35" high and is 3 weeks into flowering. In the last week and a half her leaves starting from the bottom have all begun to turn bright yellow and fall off. I did not feed her any nutes during veg and have not fed her anything during flowering yet. The soil she's in is this http://www.planetnatural.com/product/sunshine-mix-1/
has been since the beginning.

She gets full sun 80% of the day or at least some filtered sun through a tree the other 20.

The yellowing of the leaves originally began at the bottom, but today I returned home and it's skipping whole nodes all together and affecting 1 / 2 of the fan leaves on the lower nodes.

Will keep an eye on it for any edit I may need to make to help with more info.

Feed it some nitrogen, depending on how bushy she is she might of cleaned your soil of N already and has started to pull it from the older growth to feed the new growth.

^ What he said, could also be late magnesium def - any brown spotting? If not, probably N def.

Some argue about it, but generally you want a good "fade" going around week 6 flower on indicas/quick hybrids. By "fade" I mean a mild N deficiency. Means the plant is using up what it's stored and there'll be less of a hay smell to the bud after harvest (caused by chlorophyll).

Feed her lightly to start, tapering up nutes as you go.. probably want to alternate feedings/waterings too. Then taper off the nutes when you want some fade.