Oh, just in case fast weight gain isn’t your only reason for the stretch marks, a quick google says that zinc deficiency can contribute to stretch marks. So if you want, you could get a blood test to check for stuff like that. Another site mentions vitamin E and B6 also.
Since those stretch marks are basically new, I would suggest you make sure to moisturize them
Stretch marks can be minimized faster by the daily application of moisturizer or stretch mark cream, Coco Butter, Vitamin E, Shea Butter and bio-oil. Other thing you may do to minimize stretch mark is to eat a skin-nourishing diet and staying hydrated.
I don’t know your previous diet, but just so you don’t get worried again about Soylent being the cause, the consumption of something like Soylent shouldn’t cause an instant deficiency of something it contains, so you were most likely deficient before Soylent if that is the reason.