Welcome to health care magic.
Yes, it is possible to have a drug reaction even after 3 weeks of taking a drug.
There are different types of drug rashes.
While some like urticaria
come within hours of taking a drug, reactions called maculo papular drug rash, drug hypersensitivity syndrome
, Stevens Johnson syndrome
can occur after 2-6 weeks of taking the drug.
In your case, it will be useful if I had the history whether you had taken T.Crocin plus before. If you have taken it before with no reactions, it is unlikely to be the culprit.
Other drugs like vertin and even the herbal supplements like ginseng could be the cause. If you are rash free even on continuing vertin, again, it is unlikely to be the cause.
A blood test
which includes your total count, differential count and liver enzymes could give us a clue in certain cases.
Since you didn't have any other symptoms like oral ulcers, fever, jaundice
,etc, we can rule out serious drug reactions like Stevens Johnson syndrome and drug hypersensitivity syndrome.
Hope this helped.