I wouldn't worry about the getting slower too much. It's actually very common for girls to go through a bit of a slump in mid/late high school. It especially happens if they have been training hard for a few years. I'm not sure if it's as common in sprints as in distance. But in my group I'd say at least 1/2 the girls went through a stage where they were either running slower or not improving at all. Most of them just had to keep training hard and eventually got past it.
Have you been very stressed recently? I know for some of my friends that was a big aspect of it... the expectations of qualifying for certain meets and getting scholarships were a big part of it.
I actually can't run anymore :( I went straight from the slump to developing a really bad sports induced cough. I still have no idea what it is (I successfully stumped UBC sportsmed's lung specialist) but after over a year of trying to train and ending up coughing super hard every time I pushed myself I gave up on it. I really miss the trackmeets and don't really know what to do with myself now that I'm not running 5-6 days a week. I had always planned on running through university and even in grade 9/10 could keep up with the slower university kids in meets.