They're not allowed to limit your connection speed, if they didn't mentioned that in their ToS(you hopefully read them, or your parents). If there isn't any mention about that limiting thing, then ask your ISP why they limited your connection and when they don't reset, that you will terminate the contract if they don't fix it. If that doesn't works, get in contact with the consumer safety(Verbraucherschutz) and tell them.
They're not allowed to limit you for something you pay money without asking you in forehand.