After miscarriage my first period was heavier but my 2nd period just spotting why is there any problem?

Housecleaning done: Mother Nature was doing some "housecleaning" after the miscarriage by removing extra endometrium and debris from the uterus. This is especially true if no D & C was performed after the miscarriage. The endometrium was then placed in "reboot" mode to grow with the second period and did not contain as much "material" at that point- therefore, spotting.