Stalled/ Yo-Yo'ing

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Well after a good start I seem to have stalled :(

I have lost around 40lbs now and I'm really happy with but in the last 4 weeks its not working, I've gone up 2lbs lost 2lbs over and over.

My diet seems okay around 1400-1600 cals aday. Exercise is 2x 1000 cal work outs,