Gaining weight

My doctor told me that I need to gain 5 pounds between my appointment last Wednesday and my next appointment in late April (I've already gained 6 or 7 from pre-pregnancy and I was underweight back then). I'm a small person and normally 5 pounds would be a lot for me...but not this time. I'm 3 pounds heavier in less than a week haha. Anyone have any healthy tips to keep the weight on? I've had horrible junk food cravings that I think are mostly to blame. Thank you and good luck to all the mommas trying to keep their bodies healthy and growing!