Multiple Entries on L Visa to China

I recently obained an L visa (90 days) valid until 26 May 2012. I originally intended just to go to Beijing in July, but now I would also like to go the Shanghai in Sept. This would be within the 90 day window, and my visa "Entries" line is stamped M--multiple entries. Will I have any problem with this second trip to Shanghai from the US? I thought to get M it had to be a 6 month visa. I did not specifically apply for M but that is the way it was issued.

Thanks.