Say 1000 bacteria were present when you had symptoms. The antibiotic killed 900 and was only able to weaken the rest 100. If you stop the antibiotic those 100 will recover over time and in that process gain resistance to that antibiotic. Also they can reside in your own body and get reactivated at a later date or may pass into others and this time those 100 will multiply to be 1000 and the antibiotic wont work on them as they are resistant.