Debbie had days like that too around 17 weeks. It happens, usually right before another hormone surge. The placenta is now responsible for the manufacture of hormones for the baby which is why your early pregnancy symptoms abate as you progress in the second trimester.
As your uterus moves higher, it releases some of the pressure on your bladder which is probably why you don't need to urinate so frequently. Enjoy the temporary respite, because as you fill out, the pressure will return.
Have you felt the baby move yet? If so, have you felt her daily? She may have flipped over and is now kicking towards your back where there are less nerves which makes it harder to feel.
If you are really concerned, get in contact with your midwife of the delivery suite at your local hospital. A few seconds with a doppler will confirm that all is well.