Adjustment of Status - I-485

So, my husband (USC) and myself had our interview two weeks ago. We've been married for 5 years and have 2 kids together (married in Canada). All my documents were in order, we already had the i-130 approved, and the IO complemented us on having everything so organized. She even started looking for a blank page on my passport to stamp it. However, after she came back from the copy room, she gave us a letter saying the case is held for further review and we should wait 4 months before inquiring about it. When I asked if everything was ok, she said yes, we gave her everything she needs, but she has to do one more background check and she has a lot of files ahead of us (this was in the San Jose, CA field office and the IO was an Asian lady called Lian). I haven't heard back and I'm getting worried. What could be wrong? I have a job with the US government, hold a doctorate, and am a very law abiding citizen.
I'm specially confused since even after giving us the review notice, she was still asking which picture I would like for my green card,etc. I really don't know what to think and hearing horror stories on these forums about people who have waited for years after interview really scares me...
My timeline:
i-130 approved in August 2014
i-485 sent in Sept 2014
Interview notice sent in March 2015
Interview in April 2015

Has anyone gone through something similar? How long did you have to wait after the interview?

Many thanks!