Why My Period Is Late?

hello, my name is cassandra, im 18. i havnt had my period in 3 months but for the past year and a half my periods have been 1 months or even two months late. i thought i was having my period but what ever it is comes out dark and thick. i have taken at least 3 pregnancy test and they all came out negative, i have always had safe sex. is there a possibility that i could be pregnant?