"Why do the front of my ankles hurt after I've been en pointe?"
Good question, FYPS (that would be First Year Pointe Student)!
Most likely answer #1: You are not lifting with your core and hips, but rather solely your feet. When you don't utilize your entire body to get yourself onto the tops of your pointe shoes, you are asking a lot from your feet. You are making them (and your ankles) do all the work which results in strain on the tops of your ankles.
Most likely answer #2: You are using your upper legs to lift and possibly your abdominal muscles but not the glutes (your derriere), your back and the small muscles underneath your derriere, your deep rotators. This results in a pushing, rather than a lifting, especially if you are working facing the barre. You end up rising on top of your pointes and pushing your hips forward which, again, results in strain on the tops of your ankles.
Most likely answer #3: You are using your core and your upper legs and your deep rotators but you are not using your demi-plie to push the feet into the floor and roll up or spring up to the tops of your pointes. If your plie is too shallow, you end up trying to lift from the top of your feet instead of the bottom.
So, to avoid that pain, always use your plie, press your whole foot into the floor, engage your core and upper legs and rise en pointe with your whole body, not merely your feet.