Hi, Congratulations for the weight loss!
Anything that causes stress to the body, including very intense exercise and calorie restriction, can disrupt the menstrual cycle but if you have been sticking to a reasonable calorie allowance (i.e. above 1400 calories) then it's probably less likely to be caused by dieting. If it continues then it might just be best to see your GP to discuss the problem and s/he should be able to tell you if it's likely to be diet-related. Oh by the way, in general Slimming World suggests that you reduce your fat and dairy intake but for a healthy menstrual cycle you really need enough healthy fats (e.g. use the occasional teaspoon of olive oil in place of fry-light, have a little bit of oil-based margarine on your toast, eat oily fish or take omega-3 and evening primrose oil capsules).
(PS. The NHS website suggests taking agnus castus/chasteberry for PMS and when I looked up reviews on Holland & Barrett a lot of people said it helped their cycles return to normal after they had been disrupted. Also taking calicum, magnesium and B6 supplements alongside evening primrose oil might help)