It sounds like you’re low on electrolytes (the legs seizing up issue), and on carbs (the shaking and dizziness). Maybe dehydrated too. It’s possible that you’re running too hard/fast but also that you aren’t getting everything your body needs.

Just as an experiment, try to bring a Gatorade or something with you next time (NOT sugar free), and drink it while you run. It might help a bit 😊