Day 20 on new level of Lustral?

Hi. me again, I am on day 20 of new level of Lustral/sertraline & was slowly getting there, but the last two days my anxiety has got bad & I am feeling sick. Just wondering if this is normal, I also read it takes 4 to 6 weeks for it to work on anxiety, but I have had good days then bang. Thank you for your thoughts.