What the heck was going on?

My guess? Your metabolism was revving up to adjust to the much higher levels of exercise and your kidneys were working harder to remove increased amounts of metabolic byproducts, like uric acid, that your muscles were producing. By the fourth day out, your systems were all starting to function more smoothly and you were excreting your metabolites more quickly, during the day and evening. Like I say, just my guess.