Really confused why i am not losing weight this year. Advice needed please.

So i am a 28 year old woman around 5'5" and currently 140lbs with a goal weight of 126lbs. I am a nurse so although it's not really a physical job, i am on my feet all day and occasionally lifting patients etc.

Every year i am around 140lbs in January and gradually lose around a stone to get down to around 126lbs before my summer holiday in June, and then gradually get back up to 140lbs until after Christmas. I've done this pretty much every year for the past 5 years.

Last summer i got down to 124lbs, one of the fittest i've been (for my wedding), but by January this year i had gotten up to 150lbs, probably the biggest i have been. The first 2 months of the year went really well and i went from 150lbs to 141lbs. However the past 10 weeks i have only lost around 1 pound in total and am really confused why the weight has stopped coming off. I have not changed anything and done everything the same as i normally do - gym 4/5 times a week and a pretty strict diet. Every single day i am eating less than the calories needed to maintain my weight and once or twice a week i limit myself to 800 calories a day. Some weeks i have done this i have even put on weight.

I go on my yearly summer holiday in 5 weeks and am way off target this year. It is confusing me as i am doing the same as i do every year which has worked for years. I am just after any tips / advice of anything i can change, as for whatever reason this year the weight has stopped coming off and it is so frustrating and upsetting having lost only 1 pound over the past 10 weeks given how hard i have worked with my dieting and at the gym.

I have been considering the 5-2 diet as i have heard people have success with it, but at the same time i am reluctant to as if i am not losing weight on a strict 7 day a week diet, i can't see how i would lose weight when eating regularly for 5 days a week.

Thanks for any help.