26weeks pregnant. Why does it hurt to walk?

Sandra
  I am 26weeks pregnant and i am trying so hard to walk but as soon as I start walking well like 10-15 minutes after I start, my lower belly starts to hurt.
   I get a sharp pain and it forces me to stop and breathe and wait for it to go away. It kinda feels like a cramp.  It use to happen when I was pregnant with my first too. Does anybody know what causes that pain?