Confused??

Alyssa
So I'm on birth control, and I have been consistently since March/April. I did not get my period in July and now I didn't get it in August either. I'm technically 2 weeks late from my second missed period. I've taken 6 pregnancy tests and every single one came up negative. I'm not sure if I should be worried or what... How many missed periods is too many while on birth control? 
I haven't had any pregnancy symptoms either. Just some lower back pain, but I think that's from sitting all day with my desk job. 
I'm just confused and if anyone has some advice, I'm all ears.