Since cutting out junk food,iv gained weight and unable to lose it.HELP!

Im 19 and I had an eating disorder,where i starved myself but stopped about 5 years ago. My weight has been pretty consistent since then,but i was eating a lot of junk food. Probably about 3 times a week,and a bar of chocolate daily. 6 months ago i decided to cut out the take away,but kept eating the choc daily. I found i was eating a lot more,probably to replace my junk food cravings,and i did eat a little more chocolate than usual. The problem is i have now put on 5 kilos,basically only on the top of my thighs and i have a huge spare tyre on my stomach,and i cannot get rid of it. Ive been trying for the past 2months and I just joined the gym,so i have started exercising recently. I just want to know how its possible that iv GAINED weight from cutting out that crap?! And will i ever lose it?It seems hopeless. I usually drop any weight i gain in a few weeks. Also,lately in the past couple of weeks iv been constipated and i havent had my period for about 3 or 4 months now. And i am not pregnant haha