I'll be 5 weeks tomorrow (2nd pregnancy) and am experiencing the cramping this time around. Mild cramping is completely normal as its your body adjusting to the changes of pregnancy. The cramps can also be followed by light spotting/bleeding, don't be alarmed if this happens. And lower back pain, also completely normal.
If something doesn't quite feel right or you want to feel at ease, have a chat to your GP or midwife x.