It is, but if it was likely you'd probably already know - like if I gained 8lbs at once it would probably be mid-exams and I'd know exactly how and why.
So - were you buck naked? If not, were you wearing more layers than a month ago?
Was it the same scale?
Did you do more than one reading on the scale each time? (I always do, because I've had scales that read differently within seconds of each other). If you don't take more than one reading, you'll assume the weight is accurate (or close to), when the darn thing may actually be broken.
Is it around 'that time'?
Could you be bloated?
Do your clothes fit the same? If not, where are they tight? Where do you usually put on weight?
Have you started any time of exercise lately (could be as simple as suddenly walking more, more housework, raking leaves, etc. - you could be building muscle mass).
Bear in mind that a number of these things could be happening at once - and all together have a more dramatic affect than you would normally expect.