Your body has two circulatory systems: one for blood, with arteries taking fresh blood from the heart to various places and veins taking it back, and the lymphatic system, where clear fluid sort of seeps around from the upper body in the spaces between muscles and organs and is returned by vein-like passages studded with “lymph nodes”, which are basically one-way valves.
If you’ve ever skinned an arm or leg and had clear gunk oozing from the scraped area, that was lympatic fluid.
Sometimes the lymphatic return system doesn’t work well; if you see someone whose feet and/or lower legs swell up when they stand very long, it’s because lymph is pooling down there and the lymph nodes aren’t raising it up any more. The condition is called “lymphedema”, and doctors don’t know why it happens. Well, there are a bunch of them who are 100% sure, but they don’t agree with each other much.
Lymphedema can also be a side effect of some radiation treatments, like for breast cancer. In that case, the arms and shoulders might swell up.
Jelquing and stretching stress the tissues of the penis. You can damage your urethra and piss blood. That’s not unusual. You can build so much pressure you get petechiae(red spots) or bruises on your glans; that happens to almost everyone until they learn not to pull so hard. The arteries and veins seem to be remarkably resilient, but the lymphatic vessels sometimes don’t like it at all. It doesn’t happen to everyone, but it happens once or twice to some people, often for a small number.
The lymphatic vessel will turn hard and stringy, and sometimes painful. This is attributed to “gunk” of some sort obstructing the vessel, but as far a I know nobody has actually had surgery to go in and look to be certain. It acts like an obstruction; sometimes you can stroke along the vessel and free it up, or use a penetrant like Eroset’s Vein Oil or DMSO, or just wait and it will almost always clear up eventually.