Aiyoh, at 13 weeks ini memang normal lah, your uterus is growing and sometimes you'll feel it tightening (we call it Braxton Hicks but honestly at this stage it's just your body adjusting). A few of us had the same thing and our gynae said it's nothing to worry about as long as tak ada bleeding or severe pain. That said, since you're asking, better check with your doctor next appointment just to put your mind at ease, they can confirm everything is progressing well!