So I've been researching this A LOT online and looks like it could be water retention from having drastically increased the frequency and intensity of my workouts.. Going from doing little exercise apart from walking/the odd gym visit to two and a half hours per day of intense cycling literally overnight has caused muscle tears all over my body and a bad case of DOMS so my body is retaining water while my muscles are being repaired, causing the soreness I feel all over and the weight gain. Also there is the increased glycogen storage. Seems to make sense and the sources I have read say it's totally normal when starting a new exercise regime and should go away in a couple of weeks time.. Phew! I'm going to keep cycling
Hope the above explanation helps someone who may be in the same (distressing) position and reading this