Well instagram doesn't allow nudity to the best of my knowledge, so what do you mean? What do you mean by provocative?

If you mean like, a 14 year old posing in a provocative but fully clothed manner, that alone probably wouldn't count as child porn... however if you had multiple such photos of different teenagers, then that could certainly qualify. When police or prosecutors look at such situations, they are almost certainly going to try to figure out what the persons intent was. If they have a bunch of provocative but clothed photos of young teenagers, the police are probably going to assume they were "using it as porn", thus, making it child porn.