"must" implies certainty that something is true; obligation; necessity. "can" implies ability; possibility. You MUST be a bit tired. (= I am very certain that you are a bit tired.) You CAN'T be a bit tired. (= You are not allowed to be tired. or I don't think it's possible you could be a bit tired.) You MUST be so tired that you had to stop and rest. (= I am very certain you were so tired that you had to stop and rest.) You CAN'T be so tired that... (= I don't think there is a possibility that you are tired.)

The second sentence expresses disbelief and a sense of scolding: I can't believe you're so tired already that you have to stop and rest.

In the first sentence, the person is saying "I think you are tired. You should stop and rest". (in other words "You look very tired and you're walking very slowly, so you must be tired".) In the second sentence, the person is saying "I don't think you are tired. You should not have to stop to rest". (in other words "You look fine, you're seem filled with energy, so you can't be tired".)