Wa'alaikumassalam sis! Sounds like you should really see a doctor soon lah, even if it's private clinic, breathlessness, heavy breast feeling, and back pain together with a missed period is something yang kena check properly, bukan boleh teka-teka je. A few of us had similar symptoms and turned out to be different things, so doc really needs to do a proper exam and maybe blood test. Don't stress yourself lah by guessing, just book an appointment at a private clinic on your lunch break or weekend if klinik kerajaan susah, worth it for your peace of mind!