Ahhh, that's pretty normal at 28 weeks honestly, a few of us went through the same thing, those sharp pains down there boleh memang bikin takut! It's usually your ligaments stretching or Braxton Hicks contractions, tapi sebab it's coming and going, I'd still check with your gynae just to be safe and get peace of mind, lagi-lagi if it gets worse or ada discharge aneh-aneh ya.