What you're describing is usually about pelvic floor muscle tone, not a structural change, and there isn't a medicine that tightens the vagina. This is a very common concern after sex or childbirth.
Next Steps
Regular pelvic floor (Kegel) exercises done daily often improve tone and sensation within a few months. A gynaecologist can check you and guide you toward pelvic floor physiotherapy if exercises alone aren't enough.
Health Tips
Avoid vaginal tightening creams or gels sold online, since some irritate the tissue. See a gynaecologist in person for a proper evaluation.