I haven't experienced that exact sensation myself, but at 20 weeks there are so many things happening with your body stretching and shifting that odd sensations are pretty common. That said, if it's been going on for a week and it's something that's worrying you, might be good to check in with your gynae or PD just to be safe and put your mind at ease. They'll be able to feel and assess what's going on much better than we can online.
Could it be heartburn? If it’s discomfort or painful, please consult your gynae.