Aww, I understand how worrying that must feel, especially when you're only 16 weeks along, a fast heartbeat can definitely make you anxious. This is something best checked with your doctor or midwife soon, as they can listen to you properly and make sure everything is okay with you and baby, because sometimes pregnancy hormones and increased blood volume can cause this, but it's better to be sure. In the meantime, try to rest when you can and stay hydrated, as dehydration can sometimes make your heart race a bit. Take care of yourself, ya!