If ultrasound says i'm 23 weeks pregnant! when did i concieve? is it possible to have conceived 19 weeks ago?

Not impossible: Us dating is fairly accurate so the likelihood is that 23 weeks is close to the date. I doubt that it is a month off, more likely a week off.

Not likely: Ultrasound measurements for dating in the second trimester of pregnancy(14-28) weeks are generally accurate to within 10-14 days in most cases. Accuracy can be affected by experience of technician doing measurements. It is possible to be off more than that but it is unlikely.