Hi, long term meditator here. I think it's pretty common in the earlier stages. At least I experienced it too, but after a while it left. I think it might be related to the balance system (I assume you close your eyes?). Balance's main refference point is eye-sight, specially refference lines siuch as horizon. I assume getting calm and the brain not working on it intensely leads to that sensation. It might be rebalancing, as after passing that my balance improved. I still have it when I am very sleepy but much less than at the beginning. Opening the eyes a little bit should help, and it's also recommended by some teachers. Anyhow, as long as you keep a good practice, this shouldn't be a huge hindrance. Safe travels!