For a teenager to run a sub 2 minute 800m is really really good.
for some one to improve over 5 seconds in a span of a year is doable if you didn't have a proper coaching until a year ago. Once you get on a "real" training program where everything is minutely controlled, its not unconceivable to show that kind of improvement in a year. Something Semenya did. It also sounds like she didn't have proper training prior to last year when she burst onto the scene
Didn't watch the race but if she didn't completely blow apart the field, then it doesn't sound like a foul play. If she was ahead of the pack by two seconds but the pack was spread more than two seconds, then there really is no problem. Whereas if she was two seconds ahead and the pack was tight meaning not separated by more than a second or so then it may point to some sort of problem.
If she didn't break the world record at 18, it doesn't look like any foul play.