Nausea came back 18wks

I had extremely bad nausea until week 16 and had to take meds 3x day. From week 16 I started feeling better and in week 17 nausea was only in the evening. Now I'm in week 18 and strong nausea came back and have to take meds again..is it normal? Did you also have sudden return of nausea in week 18? I'm exhausted.