Like the last poster said your level just may not he high enough. Best thing to do is get a blood test. But honestly the depo shot can competely mess up your cycles and make some pretty funky things happen so I wouldn't rule that out either.
I missed my Period in September and took a test Sept 28th and it came back Negative...I was sure I was Pregnant..I felt all these Symptoms, missed my Period, everything! October comes along...still NO period and feeling the same...take a test Oct 29th and got my Positive. I was more than 6 weeks before I even found out I was Pregnant. I had conceived Sept 5th.
My doctor couldn't give me a reason for getting negative tests when I was very Pregnant. Some bodies just dont produce enough HCG...or fast enough for that matter. And some bodies just don't like HPT's! I know mine doesn't! haha. So it's definitely possible you're Pregnant...I would get your blood taken, the numbers can't lie and its less stressful..instead of waiting week after week taking test after test...
Good Luck, keep us posted!
take a test again and see what it says and go from there. if your preg it was from befor the shot