I’ve been taking sertraline for 21 days and have been really unwell on them.

First off, as you know everyone reacts differently to medications. Having said that most people who are prescribed this Antidepressant feel improvements within the first two weeks of starting treatment with this med, this has not been your case, but you should start feeling better soon. Hang in there, as this antidepressant is widely used because it is effective. For Sertraline to reach its full therapeutic effect it takes 4 to 6 weeks and in some cases up to 8 weeks, so again, be patient you will get there.
You do not mention what dose you are on and it would be a good idea to contact your DR., perhaps all you need is a dose adjustment.