My daughter's growth spurts were like that from day one. I have seen some kids really get pudgy as they hit puberty as well. I would not think your daughter has hit puberty at age 7, but there is a thing called precocious puberty, if I am not mistaken.
Have you never noticed her growing out before shooting up before? My daughter ALWAYS has her clothes get a little snug around the middle before they become too short overnight. Usually with her, it is in the spring/beginning of summer and living in a warmer climate it is very noticable b/c that is the beginning of swimsuit weather. But usually, with a month or two, she has trimmed right back down (or rather, actually, shot up a little so that she is back to her proper proportions).
I have known girls to get really round around puberty, and some lose it gradually, some faster, and some stay a little rounder. My daughter is 12, and hasn't had any dramatic roundness happen, though she is maturing physically in other ways.
Son, on the other hand, got rather pudgy for the first time ever in his life, when he was about 11. It lasted around 18-20 months and then he began shooting up and trimming right back down to the washboard abs he always had. He's 15 now and just as flat a stomach as ever. That little weird time around 6-7th grade was the only time he was pudgy like that. Little waist rolls and face chub. :D
If it isn't the normal pattern for your daughter, then I would ask her pediatrician about it at her next wellness visit. Perhaps she is eating more calories than you realize (clean or no... calories count). Or it could be a new (but normal) growth pattern for her.
If it is all in her belly (not face, or anywhere else) then def consult with the pediatrician. Belly fat is the most dangerous of all body fat. The fat is stored in the organs rather than just on the outside of the muscle layers. Better to check with the experts in case there is something amiss.