Based on the symptoms described, you appear to be experiencing antibiotic-associated diarrhea (AAD), which occurs in about 20% of patients taking antibiotics.
Persistent diarrhea for 18 days post-antibiotics is little unusual needs further work up Basic ones like CBP,
CRP,TAH,USG abdomen
If needed ugi endoscopy and colonoscopy at later date after consulting ur gastroenterologist