I'm Struggling With Gaining Weight After Having Irish Twins (now

I'm struggling with gaining weight after having Irish twins (now almost 2 & 3). I've added protein drinks, snacks w nuts, etc. still not gaining. I did my have my iron and thyroid checked, due to fatigue issues. It's fine. I had someone ask if I was diabetic.... Could that be my problem? I never considered it and the doctor didn't mention that. Would that be a separate test.