How many times can I use my Visa if it says Multiple Entries?

It sounds like a pretty silly question, but I would like to cover all my bases just in case. I recently got a Chinese entry permit in past December 2014 and went to go visit my mother in Beijing in January 2015, went to Japan for the weekend to visit my cousins, then went back to Beijing and came back into the States in February. So, I'm planning to go visit my mother again at the end of May (next month) and wanted to make sure my visa is still valid. It does have an (M) listed under Entries, but did my two prior entrances into the country count as my multiple entries? Also do I need to do anything else prior to leaving? Do I need another formal invitation?
Thank you- Jordyn