This is a .70 sized 4 stroke engine that I have been using for the last 3 months. I can always easily start it by flipping the propeller backward with a chicken stick. Today, after I started it for my second flight (using the same trick), it was turning the propeller backwards. The airplane was trying to move backwards. I stopped the engine. I restarted the engine with an electric starter, and everything was back to normal. I made a couple of flights after this incidence and everything was normal.

Anyone had similar experiences? What caused this? Should I be concerned?

Thanks in advance.