Thanks for your query.
Since you had the depo shot and its less than 3 months, that should protect you against pregnancy
right now, though every method of contraception
has its own failure rate, however low.
Depo shots also cause irregular menstrual bleeding and scanty bleeding, so you could be having an altered period because of that.
The cramps and backache
However, you had unprotected intercourse
BEFORE your depo shot, and although it was the last day of your period, chances of pregnancy are there.
So take a pregnancy test to be sure.