I haven't experienced this myself, but I do know that second trimester bleeding can be quite unsettling even when it's painless. Since your gynae has already ruled out the main concerns like subchorionic haematoma and placenta issues, it might be worth asking if the progesterone could be playing a role, or whether they want to monitor it more closely at your next visit. I'd probably just keep a note of when it happens and how much, so you have something concrete to discuss with your doctor if it changes or increases. Wishing you a smooth rest of your pregnancy.