You were treated for stomach infection with H.pylori, which is the underlying cause for gastritis and ulcers in most of the cases.
Although, the infection is usually easily treated, still, there are cases of failed treatment.
If initial antibiotic treatment fails, another attempt must follow and if necessary a third or fourth until the infection is cured in 100% of cases.
Since, a clarithromycin based regimen was used a metronidazole based regimen should be used afterwards.