This should clear things up.
Martin, if there had been no tradition of belief in the Blessed Virgin Mary’s Assumption prior to 1950 when is was infallibly
proclaimed (not made up) by Pope Pius XII, then it would be fair to accuse the Catholic Church of inventing doctrines out of thin air. As you can see from the link provided, however, this is not the case. The dogma has quite a long tradition of belief in the Church. But my guess is you’re not all that interested in being fair to the Church, as finding this information would have taken you a mere 30 seconds or so on Wikipedia or some other halfway reliable source.