What Does Thickening Of The Uterus Lining Mean?

had my period on the 10th to 14th of january i have irregigular periods also,me and my husband had sex on the 26th of january and the 1st and 2nd of febuary,well on the 8th i had spotting and completely stopped bleeding since,i had a pregnancy test done at my doctor office the 9th and it was negatove, well thw 10th i had really bad cramping and i wemt to the hospitalthe doctor done a blood and urine pregnancy test and a pelvic exam and ultrasoud the pregnancy tests came back negatove but he said i could still be pregnant just very early on. Because he said that my uterus linning is thickening back up. And told me to wait a week and test again,what does that mean i am pregnant oe what