What do contractions feel like?

Su
Hi ladies
To anyone who has already given birth, I'm just wondering how you knew you were in labour? I've been having pretty strong period type pains on and off for a while but only in my lower abdomen/pelvic region. Could these be contractions? I read online that contractions cause pain in the lower back and I don't have lower back pain at the moment...
I'm 39w5 so wondering whether this could be it!
Thanks
S