Aw normal pa rin yan sa third trimester, mama! A few of us na preggy na rin went through the same thing, the dizziness and nausea can still happen because of hormones, the weight you're carrying, at ang pagbabago ng blood pressure mo. But since you're already 30 weeks, I'd still suggest checking with your OB just to make sure everything's okay, especially if it's getting worse or kung may ibang symptoms na nakikita mo, better safe than sorry talaga!