The things to be worried about would be looking very red or weepy, if the skin cells were inflammed (this would be obvious as it would look almost blistered, but still solid), or if it was abnormally tender or the area is hot.
Is she on any anti-inflammatories? Honey was after her spay, but the wound was still a little swollen - made to look worse by the fact that there was no fur to soften the lines of her tummy - for a day or so afterwards. Rapidly increasing swelling would be a different matter, as this could imply some internal bleeding - though this would also cause serious pain to the touch.
After what you've just gone through, it's a well to be cautious, and if you are in any doubt, do speak to your vet - after all following your instinct was what caught the problem in good time!
Hope Summer continues to recover well!
