Perhaps it's not that he disbelieves you, but rather that he feels he doesn't have enough time to thoroughly discuss your concerns.

Yes, Repatha side effects can emerge months after you start the medication. Common side effects of Repatha include body aches, pain, and muscle aches.

As for whether it could be something else entirely, it sounds like your doctor is unwilling to address it directly. You might try bringing up the common side effects listed for Repatha, like body pain and muscle aches, and point out that these are known side effects of Repatha. See what his response is then.