Delayed Onset Of Periods With Lower Stomach, Back Pain, Tiredness, Mood Swings. Pregnant?

Hi i usually get my period days 23rd-28th, since Sept. 2012 i have been haveing late periods. i dont usually get that. my periods have become more late over the months and shorter. in november i didnt get my period at all but i got it today on dec 3rd. ive taken alot of home preg. test(all negative). i have sharp pain in my lower stomach , extreme back pain , tired alot more, very moody(could be because i got my period today) sore and tender boobs, and ive noticed im getting alot more acne if that has anything to do with whats going on with my body. i have lower stomach pains thoughout the month sometimes it feels like extreme labor contractions sometimes it just feel like little cramping. could i be preg. or what suggestions do you have?