contractions only on one side

Hi all, 27 weeks today with my first. Throughout the last month or longer, my right side will randomly get ROCK hard and super uncomfortable. Never the left. I can only feel his kicks on my right side as well. Why is this? My doctor said that random tightening of the muscles are a type of contraction “preparing for birth”, but why only one side? It’s very visible too, I tried to snap a picture but this is the best I could get. Not sure if you can tell, but my right side is more prominent. The rock hard feeling will last for up to an hour, happening multiple times a day.