You're going to have to test. A test should be positive if you've missed a period. Usually, symptoms such as back ache do not occur until later in pregnancy. I am 21 weeks and I am just starting to get a sore back. And I don't think weight gain is a sign of pregnancy, at least not so soon. Weight gain shouldn't pick up until the second trimester. We cannot tell you the odds of you being pregnant, and we cannot say if the symptoms you are experiencing are pregnancy symptoms. Many times, pregnancy symptoms can mimic the signs of a period, so symptoms are not a reliable source of diagnosing pregnancy. Like I said, if your period is late, a test should be positive if you truly are pregnant. All you can do is test.