Back cramps

Kelly 👧🏼👼🏼👶🏼👶🏼 • Married 👰🏻 (2008), 1 DD 👧🏼💖(2012), 1 DS 👼🏼💙(became an angel 2/26/16), spontaneous B/G twins 👶🏼💙👶🏼 💖(2017)
Is anyone else experiencing back cramps mostly when laying or sitting? I'm 3 weeks 5 days. My cramps aren't unbearable or sharp, just uncomfortable. 
This is my second pregnancy (DD will be 3 in Oct.) and I found out I was prego at 5 weeks with her. My periods were way off when I conceived with her (PCOS).