Most likely bloating. 1 Ib is about 3500 calories (that are not used by the body) so you would have had to have eaten a LOT of extra calories in a very short amount of time to have gained 7Ibs in non-water weight, weight. A large pepperoni pizza is about 2000 calories.
I put on 10lbs before I knew was pregnant because I craved junk. I stayed away from junk once I knew I was pregnant. You cant diet or restrict yourself when you are pregnant but you can eat right. dont cut anything out. Carbs, dairy, veggies, fruit and protein. My goal (since I was overweight before I got pregnant) was to not gain any weight. I had only gained 12lbs till I was 7 months pregnant... then I put on A LOT. Im up 30lbs now in total and Im due in a week.
Everyone is different. That said, its hard not to look for comparisons with other people when you're unsure of something.
I'm 12 wks pregnant with twins and have only gained one pound - so I'm quite nervous too.
The best thing to do would be to ask your doctor and eat as nutritiously as possible. Good luck!