weight gain in women

I am a 27 year old female and i have 2 children. My youngest is 2. Before kids i was always small and never had problems with my weight. Ofcourse having kids changed that...however i learnt that after my first child i could diet and excersize reg and loose the weight. As i did with my second child. Now that has changed. I'm not on any birth controll or antidepressants or anything that could keep me from loosing weight. I stay active and do not go over my calorie count from my doctor. I have been trying for a year now to get the extra weight off but i keep gaining. I am now as big as i was when i was 9 months along with my second but doing more and eating less than i did when i lost weight after she was born. I've had my thyroid checked, b12..and had ultrasounds done to make sure that there weren't any mass swelling or anything of that sort. i've gained 25lbs in less than 3 months and i can't figure out what is going on. Any clue what this could be other than what i've said the drs have tested me for? i do drink on the weekends but other than that i don't do anything that can cause weight gain or swelling and ab pain like i am experiencing. seems as if i am always uncomfortable and cramping and feeling "full" even when i haven't ate anything all day. i can barely sleep at night and i'm always stressing about the weight. i even went to a diet clinic for almost a year straight and bought an eleptical that i worked out on regularly and still couldn't shed the any weight but i've actually gotten bigger. please help! any advice!!