Ay, I totally understand your worry kasi that sharp feeling sa lower belly talaga nakakabahala, especially when baby's moving a lot pa! A few of us mums experienced similar stuff during mid-pregnancy, it could be ligament pain, round ligament stretching, or just the weight ng baby pulling down, but honestly, at 21 weeks with that level ng pain when you're walking or standing, I'd really suggest checking with your OB para sure lang, kasi sila lang ang makikita kung normal stretching lang or something else. Don't wait pa, okay? Better safe talaga when it comes sa baby and bumps.