Brown spotting on leaves, possible bug issue

Hey everyone. I went outside today, to check on all of the younglings, and I noticed that one of the plants had these brown spots on the leaves. I am not sure if they are caused by bugs, or what not. We had some hard rain here a couple days ago, and I covered them with spare pots and buckets, so there IS a possibility that something was transferred over, however I have not had any issues with any bugs in the last few years, so I am not sure.

All of my plants were germinated on 05MAR2015

I hope these pictures can help you to help me determine what the cause is, and I appreciate any help you may be able to offer.

Extra information to potentially help

How long has this problem been going on? Less than two days
What STRAIN are you growing? Chemdawg
What was the establishing technique? (seed or clone?) Seed
What is the age of your plants? just over one month
How long have they been in the soil mixture they are in now? appx 10 days
Were they in the same mixture when they were seedlings/smaller plant? If not, what mixture were they in before? Foxfarm Ocean Forest
How Tall are the plants?
What PHASE (seedling, vegetative or flower) are the plants in? Seedling/Early veg
What Technique are you using? (SOG, SCROG etc) None yet
What size pots are you using? (Include how many subjects to pot) In Ground
What substrate/medium are you using? What brand of soil mixture are you using?(percentage of perlite, vermiculite...etc?) Soil with perlite mixed in (90% soil 10% perlite or less)
What brand Nutrient's are you using? Foxfarm and Humboldt County's Own
How much of each nutrient are you using with how much water? *Knowing the brand is very helpful* Mixed per manufacturer grow tables (at 1/2 strength) in Two Gal water for 5 plants
How often are you feeding? Every other watering (watering every two - three days, less if top of soil seems damp still)
If flowering, when did you switch over to using Bloom nutrients? N/A
What order are you mixing your nutrients? (example: veg nutes 1st, bloom 2nd ect) Veg, Bloom, Additives
What is the TDS/EC/PPM of your nutrients used? Untested
What is the pH of the "RUN-OFF"? untested
How often are you testing pH/ppm/EC/TDS? untested
What method of pH test was administered? Using Strips? pH pen? untested
How often are you watering? Every two to three days as needed
When was your last feeding and how often are you feeding? Four days ago, today would have been feed day, but after rain, soil was still wet
Has plant been recently pruned, cloned or pinched? No
Have any pest chemicals been used? If so what and when? NA
Are plant's infected with pest's? Unsure

They look great! The stuff under the leaves looks like soil splash up from watering. Get a jeweler loupe to check them closely. Add chicken coop wire around and over them or mice deer etc will eat them soon. Nicely done so far!

so here are some pictures from today. it doesn't look like bugs, but I have no idea what would be causing the leaves to react like this. none of the other plants are looking like this, only this one.

the new growth doesn't seem like it is affected by whatever is causing the older leaves to react like this, but it is still concerning to me