Don't Panic!!
I am guilty of the same and for some very strange reason, that I still cannot fathom, I always seem to have lost when I get to the chemist?!
I take it you do have an official WI, rather than going off your own scales?
If they are usually accurate, is it TOTM, have been you been 'bunged up' at all, are you wearing something heavier than you would normally?
There's a few reasons 'why' or, the absolute worst, maybe you've lost inches this week and not much on the actual 'weight' side?
Only suggestions - maybe some of the more 'experienced' LT'ers can answer better....
Fingers crossed it is evil home scales tormenting you x