HELP me!

Jecelynn
I'm always on time with my period. Always. 
I had an emergency laporotomy done on June 1, two days later (while still in the hospital recovering) my period showed up, on time and all. 
I am now 3 days behind, but I've taken two pregnancy tests and they've both come back negative.
I haven't had any of my pre period symptoms. The only new change for me so far has been an excessive amount of acne, and a much greater appetite. These two are never symptoms of my period either. What do I do???
I talked to my mom and she told me that when she was 6 months pregnant with me the sticks still told her she wasn't pregnant....