Don't have this issue but you could always tell her that you went to the doctor and they said you have acid reflux and it's making you sick. Say that you have been put on tablets to make you better but they take a while to work. I'm sure she is worried about you.
I think it depends on how bad it gets. My morning sickness with number 2 has been so bad (HG) I've been signed off work, admitted to hospital and on meds. People were starting to worry that I was really ill and I felt the lying was getting out of hand so I've felt I had to tell everyone. At one point I had to go stay with my mum for a couple of weeks as was so unwell and struggling to look after my two year old. People were beginning to think me and my husband had split up and all sorts so I just came clean. See how you go.