While the Soviet Union was, by wartime necessity, aligned with West, you should not underestimate the degree to which tensions and disputes would still emerge between them. These tensions over large-scale military strategy and decision-making were already in place long before 1945.

Remember, the Eastern Front was the most intense theater of fighting in the Second World War, as Germany's invasion of the Soviet Union ground to a halt and turned into a war of attrition. This context created one of the early points of tension between Stalin, Churchill, and Roosevelt, given that Stalin had been calling for a direct invasion of France as early as 1941 (see reference link), in order to relieve pressure on his own position. This vision, however, conflicted sharply with Churchill's own plans. As historian John Merriman writes (referring to the situation after the victory of El Alamein):

Churchill wanted to strike at what he called the "soft underbelly" of the axis through Italy, the Balkans, and the Danube Basin ... to protect British interests in the Middle East. Stalin, however, continued to insist on a major Allied attack against Germany in the west. ... Churchill, however, feared that a direct confrontation with the largest concentration of German troops might be disastrous and wanted to postpone a cross-Channel invasion of France as long as possible. (A History of Modern Europe (3rd Ed.), pp. 1088–1089)

This difference of opinion on military matters represented a significant point of tension between the wartime leaders, already poisoning the well within this wartime alliance.

In addition, you should think about the tensions that emerged over the post-war planning between Churchill, Roosevelt (and later Truman, after Roosevelt's death), and Stalin, as well as the political situation that emerged with the large-scale occupation of Eastern Europe by the Soviet Union as the war drew to a close. This growing distrust could be observed in the Potsdam Conference in 1945 (held shortly after Germany's surrender), which saw disputes over reparations and over territory between Stalin and the other Allies. Stalin, for example, wanted to acquire parts of Turkey, including Istanbul, which Truman and Roosevelt refused to accept.

Finally, everything else being said, it might be worth wondering whether the wartime alliance could have ever held together in any condition. Ultimately, there were longstanding tensions between the Soviets and the West, and in any case, communism has always been built on an ideological opposition with Western-style capitalism. (Consider, for example, the global revolution advocated by Trotsky.) With this in mind, there's a strong argument to be made that the wartime alliance was an aberration to begin with when it comes to the longer course of Soviet-Western relations.