gaining tons of weight

I've been on lamotrigine for 3 months or so and lithium for about 6 weeks(?). I've noticed I'm gaining tons of weight. Clothes that fit me just 2 or 3 weeks ago now feel tight and look dreadful. The weight is all accumulating around my midsection and hips. I'll admit that I haven't been super careful about watching what I eat, but I've never seen so much weight go on my body so quickly. I'm scared to weigh myself. Weeks ago when I started the lithium, I weighed 138 pounds. It devastates me to think how much I must weigh by now. I would bet around 150. I've never gained so much weight in so little time. And I've got a terrible history with my weight. I've had eating disorders in the past and so seeing this happen makes me extremely sad. I'm considering going back to my old habits because I can't stand to look like this. I just don't know what to do. For the past 3 days I've been trying to watch what I eat, but I only see myself getting bigger. Usually I can take off weight if I am dedicated to it, but I'm beginning to become upset watching this happen. I am beginning to hate myself more and more. I just don't know what to do. Does anybody know about this? I don't want to jump to conclusions saying lithium has made me fat, but this is so uncharacteristic. I can gain weight if I'm not careful, but it just seems like SO much. I can't remember looking like this in ages. If you have any insight into losing weight while on lithium I would really appreciate it.