Well, you can not be 100% sure before your doctor confirms it.
I'm a mother of two young girls aged 7 and 3. With my first pregnancy once i missed my period for a week I normally took the home pregnancy test. Not one but 3 and they were all positive. Then I went to my Gynaecologist and she confirmed it.
But to be honest, being pregnant is not only exiting. It is very uncomfortable and stressful as well. As that was a planned pregnancy and we were trying to get pregnant, before I missed my period just about the time I was suppose to get my period I felt the same cramp like pain in my belly as I feel before my period comes as well as tender breast. I was so discouraged that I felt a bit depressed. I wanted to be pregnant.
Another friend of mine had the same “feel like the period is coming” sensation and after words she was confirmed that she is pregnant. I did some research on the net ad apparently the explanation was that at that time the embryo is getting attached to the uterus and that is causing the feeling.
Then after the doctor confirmed I felt so happy and expectant. And that the nausea, painful breasts, metal taste, heart burn, indigestion and all another associate pregnancy symptoms arrived, but about that in another post.
The bottom line is that if you think or expect that you are pregnant and feel the same the period is coming symptoms, you may as well be pregnant.