Yes I'm trying to get pregnant, but I really don't know the best way to start, so I tryed after my period 6 days later when I was fertile
For my pregnancies, my symptoms were almost automatic, 3-5 days after I conceived I showed signs, so its possible! Everybody's body is different & handles events differently so this could be a sign, but then it might not! Are you late? Do you normally have these symptoms?.... Are you trying to get pregnant???
There is a possibility that you are pregnant but to be sure you might want to take pregnancy test to be on the safe side.
I don't believe you will have "really bad pain in my lower back" just days after fertilization. There are any number of things that could cause low back pain. Wait for your cycle to come and if it does great, if it doesn't then take a pregnancy test. And unless you were trying to get pregnant, you may want to invest in some condoms.