The chances of becoming pregnant in the situation you described is low. It is very common for patients using a new form of contraceptive to experience spotting or bleeding for the first few months of therapy. Many people unfortunately experience spotting for up to 6-8 months after starting a new or different contraceptive! After taking the injectable, you should have full efficacy of the drug within 3 weeks of the dose as 3 weeks is the time it takes for peak concentrations to occur in the body.