What Causes Delay In Periods,weight Gain And Discomfort In Abdomen?

My age is 29. my regular period cycle is of 28 days but I had my last regular period on 7th Dec, on 27th dec I had viscous brown mucus/blood discharge for 3 days without any cramps (bad cramps are always there in my periods. Then I had light spotting red color for three days from 17th Jan to 19th Jan. I had few HPT, came back negative. I went to see a Gyn. on 5 Feb, she sent me for a blood test, it also came negative. Now if I consider that 17th Jan bleeding as my periods, m again late for my periods by a week. is this possible that HCG dint show in the blood test or is it a case of cyst? should i go for ultrasound?Other symptoms include back ache, weight gain, fluttering and swelling in abdomen, pinching sensations on sides of abdomen, lightheadedness.