At some point during the 20 weeks I put on 3 pounds one week, then nothing for the next 2, then at around 30 weeks lost 4 pounds in a week - only to put 4 on the following week. So I started weighing myself daily for a bit and it really can vary day to day, for me at least, by a couple of pounds. No idea why but I'm at around the right place for where I'm supposed to be ( 34 wks)
If you're worried, definitely check with your midwife though!
Is your face also swollen? That can also be a sign of pre-eclampsia. Do you feel well in yourself? Not everyone gets all the signs of pre-eclampsia. If you have any concerns please contact your midwife.