How does Dr. Jekyll's behavior change after Mr. Hyde's disappearance? Why does Mr. Utterson go to see Dr. Lanyon?

Once Mr. Hyde disappears, Dr. Jekyll's health improves, he feel like he is free to mingle in society.  A great weight has been lifted from his shoulders.  He begins to go out and see people, particularly Mr. Utterson and Dr. Lanyon.

When Mr. Utterson goes to see Dr. Jekyll at his home, he is told that he is no longer seeing callers.  Mr. Utterson then goes to see Dr. Lanyon to ask him why Dr. Jekyll has isolated himself and won't see him.  He finds Dr. Lanyon very ill.