Ay, first time mom ka talaga! That heaviness sa lower belly around 6 months is pretty common, kaya ng baby weight plus the stretching, pero honestly better check with your OB just to be sure it's nothing else, lalo na kung may pain or bleeding pa. A few of us moms experienced that din, some kasi nag-rest lang more, iba naman need ng support belt or pelvic exercises, so really depends kung ano ang cause. Hugs, alam ko nakakakabahala lalo na first pregnancy!