3 days ive had a heavy or pressure feeling from under my breasts down to my pelvic area low back pain too and i'm nauseous. what could cause this?

Get a pregnancy: test now. If negative, wait a week, then check another pregnancy test. If negative again, wait another week, check another pregnancy test. Don't convince yourself that you are not pregnant until you have had 3 negatives in 3 weeks and your period has returned. Otherwise it could be obstipation from gas or indigestion, gallbladder problems, acid reflux, or just muscle pulls.