I had 24 hour a day sickness for most of my second pregnancy, until 6 weeks after he was born. I know the cause.
With my first, I was prescribed iron tablets at 6 weeks. They made me feel sick; I proved it was the tablets by stopping them till I stopped feeling sick, then starting them again, and started feeling sick again. My mother worked as a doctor's receptionist and she told me it was quite common, even when not pregnant, for iron tablets to make people feel sick, and to ask them to change the brand. They had patients who suffered sickness and a different brand worked wonders. So I asked and he changed brands which stopped the nausea.
Second time, the same thing happened only this time they refused to change the brand. So I was stuck with the choice of risking severe anaemia or feeling sick all the time. I lost a fair bit of blood during the birth so I had to continue taking the iron tablets for 6 weeks. I was so relieved when I could finally stop taking them!
I have no idea why they changed the brand the first time but refused the second.