Pregnancy can sometimes cause fluid buildup in the ears or swelling in the passages, so it's not uncommon to experience this kind of muffled hearing, especially in the third trimester. Since your GP has already ruled out infection and your blood pressure is fine, that's reassuring, but it's definitely worth mentioning to your gynae when she gets back to you because she'll have the full picture of your pregnancy. In the meantime, try to keep your sinuses clear if you're congested at all, and if it gets worse or you develop any ear pain, you might want to see an ENT specialist just to be thorough. Many mums find it clears up after delivery, but there's no harm in getting it checked out properly before then.