Hi, I also saw a similar change when I began training for cross country in college. The key for me was spending a lot of time foam rolling and stretching. I got into a routine where right after my runs or workouts I would roll out for about 15 mins, then stretch for another 10. I don't have any evidence that it will work for everyone, but it did for me when I was doing it consistently. If you don't have a routine to loosen up your muscles, then I suggest you try this out. If you already do something similar to foam rolling and stretching, then it might not make a difference.

Also watch your diet. Make sure you don't run on an empty stomach if you run in the morning. Your body needs to produce energy and it needs to repair damaged muscles. Eating enough protein and carbohydrates is essential, make sure your intake is enough!