If you've had unprotected sex, maybe. We can't tell you anything based on what you've said because a hundred different things can cause back pain.
it could be that your going to get your period. because that happens to me sometimes but this month i havent had any symptoms just discharge and two days late.
Is your period late? Have you been having sex? protected? unprotected? I'm sorry, but it's a little hard to base whether or not you may be pregnant solely off of back pain... If af is late, take a hpt with fmu