If the swelling of her vulva has gone down a bit, it is possible that she is experiencing a split heat... what happens is that the bitch comes into season, then everything just sort of stalls, and usually she goes into a normal season sometime soon after (if I recall, my girl came in, stalled, came in again about a month later and had a normal season). Split heats are not that uncommon in young bitches who have not begun to cycle normally. A lot of vets who do not do much in the way of reproductive medicine may be unfamiliar with this.
Standing heat usually involves some discharge, even if it is less in quantity and much lighter in color. I would not expect less swelling when she's in standing heat.
If she exhibits any signs of illness (fever, lethargy, inappetance, unpleasant odor coming from the hind end, whatever), she should be seen on the very small off chance that she has developed a uterine infection.
