Aww congrats on your pregnancy, mama! Yung butt pain at slight cramping at 5 weeks ay pretty common actually, maraming mums sa circle ko nag-experience din nun, probably from hormones and ligaments stretching na, pero dahil first time mom ka at gusto mo talaga maging sure, I'd really suggest checking in with your OB just to put your mind at ease, lalo na may specific pain ka. Alam mo naman parang ang peace of mind ay priceless especially early pregnancy, so walang masama pag mag-follow up ka.