Cause of belly pain
At around week 20 of pregnancy, the uterus will grow very quickly. Because of this rapid growth, there is a lot of tension on the muscles. These muscles keep the uterus in place. In particular, the muscles that run along the side of the uterus can cause pain. The pain feels like a sharp stinging. Especially when you are tired, you can get a lot of pain here. The pain can be annoying, for example when you walk or when you get up from a sitting posture.
What can you do about it?
When you're in a lot of pain, it's important to regularly tighten your abdominal muscles. In this way, you prevent the uterus from hanging and flopping around too much.
A hot water bottle may also help, by causing the muscles to relax.
Pregnancy pants may also help; they are comfortable and provide elasticated support for your belly, helping your muscles and letting them relax a little.
From angelababy In Q&A