It usually happens to me in the summer and I just went almost a full month only hittin the gym once maybe twice a week. This week was my first week back and i had lost 13lbs and gained a couple points in BF during my hiatus. For me it's usually a combination of things that cause me to lose focus and say fuk it for awhile. This time my body was just beat from lifting with freshly "healed" injuries I acquired last fall. Motivation just died as it tends to do mid summer alot due to the heat. I hate lifting in hot humid weather it drags me down and honestly summertime makes me wanna chill, drink beers with friends and be more social ( which I'm borderline anti social mist the time). Although i don't go about the break in a healthy way physically (although i still eat healthy just let myself cheat more)it always seems to reset my motivation and focus when I get back In the gym and my intensity increases, so mentally its just a release I need once In a while. I also bounce right back quickly so usually within a couple weeks of intense training after a month long "vacation" I'm back to where I left off. My metabolism kicks back in full steam and muscle memory is a great thing. It's like taking 2 steps forward , 1 step back and so on. I'm not shooting for a bodybuilding career or to get on stage , I just really enjoy building muscle lifting weight. BUT, there's other things I enjoy too that I deprive myself from during my months of heavy training and sometimes I need to purge, so I do , but I always come back refreshed with a re-racked attitude and motivation. Don't beat yourself up over stuff like this , I know its hard not to when you see you're self seemingly withering away all your physical hard work, but sometimes your mind and soul need a break too. Just make sure when you finally hit the point where you think "I gotta get back in the gym DAMNîT!" You grab your shlt and get back to it rather then sink into a hole.
P.S. that first week back is always a bitch, chill your ego and take it light and build the weight up the next week and so on. If diet and training is on point the by the 3rd you should be pretty much back on track. That's my story at least .. Cheers and good luck