I have been quite lucky so far, before I fell pregnant I was having physio for an irritable disc and was in absolute agony and I was warned when I fell pregnant that it may flare up again. *Touch wood* so far it has not caused me any issues thank goodness!
I feel for ya though as I know how crappy back pain is, if I were you I would phone in sick and rest up until you can see the GP, if you are in that much pain this early you may have an underlying back problem such as sciatica or something xx