I was on the 3 months injection for 2 years after i had my daughter, i stopped in 2015 and since then i haven't fallen pregnant, we don't use contraceptives and we have sex regularly, i am almost 33 years could this be secondary infertility?

Yes: The delay in return of normal fertility after coming off this method has been cited as long as a year. After that point your system should be at its pre-BC level. If you & your partner are trying without success after 2 yrs I suggest you both go in for a fertility consultation. If there is an issue with either they can work on it or explain alternative ways to get pregnant.