Is it legal to be charged with a late fee for the previous month’s rent if it was accepted with no problem during that time?

Our complex is now under new management and we pay our rent online. The previous month we paid an hour after midnight but no late fee was assessed and it was accepted. It's automatic. So, this month we were 30 minutes late, and it assessed us for this month. Okay, my fault. Then I checked our account online and they assessed us a late fee for the previous month, showing a balance due of that month. Is this legal? Also, there was some $50 fee added that cannot seem to be explained. Some "connect fee" which we have yet to get an answer on.