@Poli: The way I see it, it would change the listener's perception because, in this context, "a continuación" and "después" assume we know the previous situation, but "próximamente" is mostly used for creating expectation about something we will do or will happen in the near future.
While "no sé qué hacer después/a continuación" would give me the idea that there is a situation to be solved, "no sé qué hacer próximamente" doesn't need to fix anything, it would just talk about making future plans.
- (I lost my job) No sé qué hacer después.
- (I'd like to open a new business) No sé qué hacer próximamente.