So does it always mean your having a miscarriage when your hcg goes down by 280 in a 48 hour.

Nearly always: If your HCG is decreasing, it nearly always means that the pregnancy is not progressing normally. Most of the time, this means a miscarriage. Rarely, this can mean an ectopic (or tubal) pregnancy. You should absolutely touch base with an ob/gyn if you have heavy (more than 1 pad/hour for 2 hours) bleeding, severe pelvic pain not relieved by over-the-counter pain medications, or fevers.