What Causes Severe Pain In Legs After Practice In A Track Runner?

So I run XC and track.. I m currently in track season right now. XC season went great. Broke records, it was good & I felt good. In between XC and track season I took about 3-4 week break. It s been about 2 weeks into track season.. But I ve been running before track started to prepare my self, well before actually.. Like a couple months. Unlike XC I feel like I m getting worse. Each praticd my legs hurt worse and I feel slower. My shins are starting to hurt but not to the point where I need to stop running. What s happening? Am I doing something wrong? It there anything I can do that we ll help me do better and pratcice ? Every practice that goes by I feel physically worse. It s never happened before now.