Hi Clare,
I had this with my 1st 2 pregnancies, now 14 weeks with my 3rd, and have it again! I was using Senna tablets as they say they're safe during pregnancy & are working ok.
My midwife did tell me during the booking in appointment to drink at least an extra litre of water a day to prevent constipation & headaches - which I also seem to be getting a lot of this time around.
Paula.