Villagers can be weakened by many things, including on-the-job injuries. Level 3 medicine will not improve her condition. Medicine Tech primarily extends the villagers' life spans. The best way to determine exactly why a villager is weak is to check the game log. You can find an explanation of how to do that in this post.

Once you know why she's weak, you should be able to take steps to prevent further weakening. Taking a villager to the food to eat does improve their health - it just takes a lot of meals to see the effects on the health bar.