I tried sitting down periodically. It just reinforced how much more comfortable it was to sit than to stand.
I find it completely implausible that being thin would making extended standing harder. Flies in the face of everything I've ever read about back problems, which overwhelmingly afflict people with higher BMI. Also, I'm quite young, and have no history of back or muscle pains of any sort outside of what was brought on by the standing desk experiment.
Why is it implausible? Standing still requires muscle. There is some minimum standard of fitness required to stand for a fairly long time.
Being thin or having a normal BMI doesn't mean you're fit. It's just a useful statistical measure to find out the general fitness of a population. Youth won't help you either.
Your spine is a stack of disks. It's got no inherent tendency to stay upright, other than the forces imposed on it by your muscles (and fascia, but that's a passive stabilization system). In particular the spinal erectors, iliocostalis lumborum, spinalis, longissimus thoracis, levatores costarum/breves, and cervical muscles.
With low bodyweight, you've got low muscle mass, particularly if you're not doing much in the way of strength training. As you age, you're losing this at the rate of about 0.5 - 1% per year (past age 30 or so). Which is where strength training comes into play.
Muscle tissue and strength are developed in direct response to imposed demand. It's called the "specificity" or "SAID" (specific adaptation to imposed demand) principle.
Full-body strength training routines -- squats, deadlifts, lunges, press, bench, and rowing / chin-up movements -- will develop that muscle.
At 6'2", last time I weighed 130# was some time in middle school. By high school, as a swimmer and just doing bodyweight strength training, I was 185-195#. These days I'm at about 260, cut weight would be closer to 235-245# with 8-10% bodyfat. I'm not saying that this should be your goal, but am pointing out that there's a huge variance in individual fit body mass. Your weight might be appropriate for a long-distance marathoner (Geoffrey Muttai, among the best in the world, is about 125# at 6' in competitive shape, but that's absolutely shredded with <6% bodyfat).
Here's my suggestion: try strength training for 6-12 months. See if it changes your perceived fitness and comfort. Like it? Keep it up. Don't? You've gained experience points and it's cost you little else. I'm a fan of The New Rules of Lifting (which describes much of the science and health/fitness benefits in much greater depth) and Starting Strength (a simpler and very brutally effective lifting program) myself.
Oh, and when you get into your 40s like me, you may appreciate that muscle mass.
We’re only focusing on the BMI issue because that was revealed by the original comment. Goodside could have spikes placed on the ceiling that dug into his head only when standing. If this was the case we’d be barking up completely the wrong tree. I’m pretty sure he’s not that stupid but my point is, without being present to make an assessment we have absolutely no idea what could be going on. It’s like trying to fix your parent’s computer over the telephone with them operating it.
It's not implausible. I used to be skinny like you, everything from getting up in the morning to getting out of the shower (cold) was worse. Standing was harder. For me standing a few hours a day is not about back pain, but for better posture and more energy.
I find it completely implausible that being thin would making extended standing harder. Flies in the face of everything I've ever read about back problems, which overwhelmingly afflict people with higher BMI. Also, I'm quite young, and have no history of back or muscle pains of any sort outside of what was brought on by the standing desk experiment.