Hi Vanman2014,
Yes this is normal for any mattress (not just memory foam). There is more about the break in and adjustment period for a memory foam mattress and some of the other factors that can also affect firmness in post #2 here. You can see an example in post #3 here of another member that purchased a different memory foam mattress and discovered that even a fan blowing on the mattress (which is something that most people wouldn’t suspect could be an issue) made a big difference in the firmness of the memory foam.
Most manufacturers are well aware that how a new mattress feels will change over the course of the first few weeks so they usually have a minimum time frame before they will accept returns to make sure that their customers give a new mattress a reasonable trial before returning or exchanging it.
Phoenix