system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why after a hard workout, our bodies only ache really bad the next day?
gpt
There are a few things you could be talking about here. 

There is something called DOMS or delayed onset muscle soreness that usually shows up between one and three days after a workout, especially when you do a new exercise. We don't actually know exactly what causes it. But it is thought to be related to little tears in the muscle fibers. It is caused by exercises where the muscle is lengthening, like when you lower down from a pull up. 

There is also tendonitis. This is caused by tendons that are mildly strained, or even just irritated. As your body rests and recovers, one of the things that happens is swelling or inflammation around damaged areas. This causes pain in the tendons, as they do not move smoothly like they usually do. 

Finally, there is general inflammation in the joints.

What these all have in common is that they are not directly caused by injury. A muscle tear or strained tendon will hurt right away. Instead, this pain is caused by your body repairing and strengthening what you might think of as "general wear and tear." This causes some inflammation, which is painful.