feel like I have a UTI but urine samples keep coming back normal?

Sam
Please help! Need advice! 
I'm 17 weeks and 5 days and for the last few weeks I have felt like I have a water infection but I've had 3 lots of urine samples sent off and all have returned as normal? I've got the burning sensation and I have had cramps and pains everyday :( 
Could this be anything else? Anybody else experienced this? 
Thank you in advance