It's awkward at best. Use I haven't fallen sick since last week. In sentences with since, the main clause verb is usually in the present perfect.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
tostyle un Is this correct English?"I fell sick last week and SINCE THEN I haven't fallen sick."Is the usage of since then correct?It's an odd, unusual wording. It's too formal. It would be more natural to say "I was sick last week and since then I haven't been."
tostyle unIs the usage of "since then" correct?Yes. That part is correct. Others have already commented on other aspects of your sentence.