Weight has just shot up, eating issues...

[TheKettle]

I'm already obese and I've just got weighed and I've gained 5lbs in 3 weeks! It's not a false reading, I weighed on Saturday and it said the same.
How can someone who's already obese gain so much in such a short time? Up to now I've been slowly gaining weight for years, but it's been SLOW. I've probably gained 3lbs a year for the last 20 years.

Any advice how to get it off quickly? Or any advice as to why it's piled on like this ? I don't even feel as if I've been eating much more than usual.

My own weight can fluctuate by that amount in a week.
Don't attempt to lose it too quickly, you'll end up putting even more back on. I would suggest getting more exercise, walking whenever you can, perhaps swimming.
Have you tried WW, they are very good.

I put on about a kg (so 2 lb) a few days before my period starts. Could this explain the weight? Maybe give it another week and see what happens?

Keep drinking water.

I've just put on 6 lbs in one week according to my 'precision digital scales' I nearly fell over when I read the numbers. Not even eaten that badly.