It sounds like your Student Discount expired, so you were charged the regular rate. The Student Discount lasts for 1 year at a time, and you have to renew it. You should've gotten an email about that, but if you missed it, you can simply re-apply here.
There is a Hulu + Student Discount offer, but you have to apply for it. If you're interested and eligible, sign up here.
Hope that helps, and let us know if there's anything else 🙂