Can Thick Endometrium Cause Pain In Pelvic Area?

Hi, I last got my period on the 23rd of april 2011. pregnancy blood test shows negative. Iam having pain in the pelvic area more on the left side. took primolut for seben days. today is the third day since i stpped the medicine. scan revealed endometrium is 18mm thick. is this dangerous and does it have any connection to my periods being delayed