If you don't exercise for a long time and suddenly exercise a lot, it will lead to decreased immunity and easily lead to various minor illnesses (I had a fever for three days a few days ago). Your diarrhea should be related to this. It's normal for you to say that the abdominal muscle groove is gone. If you don't exercise for a long time, you will have fat accumulation. Because of the muscle loss and degeneration caused by long-term inactivity, even a small amount will cover up the "groove". In fact, the ditch did not disappear, but was covered with fat.

"Feeling twisted together" should be an illusion. 300 sit-ups is not much, but because I haven't practiced for a long time, my muscles are suddenly a little damaged and tired (I don't need to say the principle of muscle growth, there must be damage), and there will be soreness and pain during the recovery period. Even if the nature of abdominal muscles makes the pain feel weak, it will mislead you into thinking that they are twisted together. Don't worry. As for correcting this problem, you don't have to worry, you can't correct it at all, because the abdominal muscles are not complete muscles, but several muscles and tendons (is that the name? Maybe what I said is wrong). During exercise, the muscle gets exercise and gets bigger, but the tendon doesn't get bigger, so a ditch is formed between the two muscles, which is what we often call a "ditch". Everyone's abdominal muscles are different, and the tendons connecting the muscles are not symmetrical in pairs, which is why some people have straight and neat grooves, some people are asymmetrical, and some people are born with different heights. To sum up, don't worry about diarrhea. Get used to it and you won't get sick. After practicing for a few days and taking a day off, you don't need to correct your muscles. It's a side effect of suddenly trying to practice after not practicing for a long time. Just get used to it.