26 wks pregnant blood in urine

Hi,

Can I ask if anyone has had this problem in their pregnancies.  It's been going on for a month now, you can't see it, it's just picked up in the tests you have.  there is no infection found when sent off, so what could be causing it...  I'm worried and it's making what should be a happy time pretty miserable!  Reassurance greatly appreciated...  I have no pains, apart from severe heartburn and the urine looks perfectly normal!!  Thanks