How did you measure the increase in LBM? This requires very advanced technical equipment. My suspicion is that you have noticed an increase in muscle volume and assumed it to be an increase in muscle mass. Those are largely due to water retention and increased blood flow. They revert quite quickly after you stop exercising for about a week.
DEXA scans are accurate and readily available in most cities for about $100. Just do it quarterly or whatever.
I am getting regular DEXA scans
Does ability to lift weight also decrease in about a week? I was recently out of town for over two weeks and came back with the ability to lift roughly the same amount I was able to prior to leaving.
My DEXA scans seem roughly correlated with the amount of weight I can do in my regular sets, which has increased about 50-70% depending on which muscle group you are talking about.
This is with heavy resistance training 3 times a week and Pilates once a week.