What Causes Light Menstrual Bleeding Along With Swollen Breasts And Nausea?

Hi Doctor. I believe I am pregnant and conceived on April 29. My period was due May 10. It was late until May 16. When I bled it was first brown and then got red and a little bit heavier. The second day was barely any blood and light spotting. It s day 3 and spotting is getting even lighter... My usual period I usually soak up about 8 pads in one day, so this is very unusual. I have taken pregnancy test and also went to the hospital on April 16, and when they did a urine test and blood work it showed I wasn t pregnant. I am feeling extremely nauseous in the morning, my breasts are swollen, and also my stomach is tender and I am feeling discomfort in my side throughout the day but no pain...does this sound like pregnancy to you, and if it s not pregnancy what could be making my breast swollen, stomach firm, and nauseous all at the same time as a late and light period? Thanks for your time I greatly appreciate.