Feeling a sensation of something breathing in your belly at twelve weeks of pregnancy while lying on your back can be normal. As your baby grows, you may start to feel movements and flutters known as "quickening." These sensations are often described as similar to bubbles popping or butterflies fluttering. However, it's important to note that every pregnancy is unique, and the timing of feeling these movements can vary from person to person. If you haven't had a scan yet, it's always a good idea to consult with your healthcare provider for reassurance and guidance. They can provide more specific information based on your individual circumstances and help ensure everything is progressing well with your pregnancy.