Ay, tummy pain on one side can be from maraming things, gas, ligament stretching if pregnant, or sometimes ovulation pain kung hindi ka pregnant, pero since it's nagging you, better check with your OB or doctor na lang para sure, especially kung tumatagal or kumukumot na talaga. One of my mum friends had the same worry during pregnancy at it was just round ligament pain, pero need pa rin ma-confirm ng doctor to be safe!