Welcome to HCM.
Your last period was on 1st August and took pill up to the 10th august after that you got another period on 14th August.It was withdrawal bleeding and not actual period.
You did unprotected sex at the end of August & beginning of Sept and started Depot Shot on 8th Sept after getting negative pregnancy
Depot shot contains DMPA and is very effective birth control method.Effectiveness is up to 3 months.Chance of pregnancy is very low more likely less than 1% per year.
You may confirm your pregnancy by doing blood test
after 7 days of your missed period date.Also you have to concern about several side effects like irregular vaginal bleeding
for first 6 to 12 months,sore breasts,change in appetite or wt gain etc.
Always consult with your doctor if serious problem persists.