I have lower back pain, tender nipples, cramps, feeling tired for no reason

It could be a variety of things. We can't diagnose remotely. You need to see a doctor.

If you think you might be pregnant, I assume because you have missed a period. Then take a home pregnancy test.

Go the pharmacy or drug store and buy a pregnancy test. Dr. M

How late are you? The symptoms you describe could be anything, which is why we can't give you a diagnosis online. Not even a doctor could do that.

If you are pregnant we would need your urine or blood in order to tell you for sure. Not possible to do on a computer.

A pregnancy test is your first step. Make sure to use first morning urine.

Good luck.