We are using TrueSkill.
If you just uploaded a new bot, what's likely going on is the following: TrueSkill tracks two statistics, mu (estimate of skill) and sigma (estimate of uncertainty). Your score (and thus rank) is the conservative estimate of skill, mu - 3 * sigma. If you are losing games, your mu goes down, but the system also gains confidence in your skill estimate, and sigma goes down. Thus, your net score might actually go up.