help period is 23days late

My period is 23days late. and my periods are very regular I get them every 29days for 4 days. I have took 6 pregnancy test and only 3

of them can out very light positives and I went to the doctor last week and had a urine and blood pregnancy test done both negative .and I have 1 child who is 4years old and when I was pregnant with him I didn't find out I was pregnant till I was 3 almost 4 month pregnant .....what should I do??? My Dr said I'm not pregnant but I have cramps and my boobs look different and no period or bleeding/spotting at all .my Dr said he can have me get a pelvic ultrasound .. what should I do??