Pregnant or not?? am so scared?

Had at end of december, I am on the pill but i had to use morning after pill due to a contraceptive mishap.. anyways 2 wks later i had a bleed at what would've bn time of period that was on Jan 6th.. I had a period again on feb3rd but it was really light. I have put on some weight and my have got a bit bigger.. I am having terrible back pain as well.. I have taken a test today and it was negative which would be 9 wks since .. I have to go for an mri scan on thurs and am seriously panicking. Please help me..